Thomas Boston: Faith in Christ Leads to Repentance...

What comes first?

"But withal, it is to be remembered, that the true way to deal with a hard heart, to bring it to this temper [i.e. repentance], is to believe the gospel. As ravenous fowls first fly upward, and then come down on their prey; so must we 'first soar aloft in believing, and then we shall come down, in deep humiliation, sincere and free confession, and true repentance, Zech. xii. 10. " They shall look upon me whom they have pierced, and shall mourn. "Therefore the Scripture proposeth the object of faith, in the promise of grace, as a motive to repentance, that by a believing application thereof, the hard heart may be moved and turned, Joel ii. 13. "Turn unto the Lord your God, for he is gracious." One may otherwise toil long with it: but all in vain. "Without faith it is impossible to please God," Ileb. xi. 6. and therefore, impossible to reach true humiliation, right confession, and sincere repentance, which are very pleasing to him, Jer. xxxi. 18, 19, 20. The unbelieving sinner may be brought to roar under law horror; but one will never be a kindly mourner, but under gospel influences. When guilt stares one in the face, unbelief locks up the heart, as a keen frost doth the waters; but faith in the Redeemer's blood, melts it, to flow in tears of godly sorrow. Hard thoughts of God, which unbelief suggests to a soul stung with guilt, alienate that soul more and more from him; they render it like the worm, which, when one offers to tread upon it, presently contracts itself, and puts itself in the best posture of defense that it can: but the believing of the proclaimed pardon, touches the heart of the rebel so, that he casts down himself at the feet of his Sovereign, willingly yielding himself to return to his duty."

Thomas Boston, A View of the Covenant of Grace.

a Christ for You and for Me...

"Now preaching ought to have the object of promoting faith in Him, so that He may not only be Christ, but a Christ for you and for me, and that what is said of Him, and what He is called, may work in us. And this faith is produced and is maintained by preaching why Christ came, what He has brought us and given to us, and to what profit and advantage He is to be received."

Martin Luther, Concerning Christian Liberty

Michael Horton on Union with Christ...

"Union with Christ is not to be understood as a “moment” in the application of salvation to believers. Rather, it is a way of speaking about the way in which believers share in Christ in eternity (by election), in past history (by redemption), in the present (by effectual calling, justification, and sanctification), and in the future (by glorification). Nevertheless, our subjective inclusion in Christ occurs when the Spirit calls us effectually to Christ and gives us the faith to cling to him for all of his riches...

"The motif of mystical union has often been presented as an alternative to the forensic (legal) motifs of redemption, especially vicarious substitution and justification. Since Albert Schweitzer, the thesis has repeatedly been advanced, refuted, and then advanced again that justification is a “subsidiary crater” in Paul, while the real central dogma is mystical union. Reginald Fuller notes, “Attempts have been made to pinpoint some other center or focus for Pauline theology, such as ‘being in Christ’ (Schweitzer) or salvation history (Johannes Munck).” However, “Romans, the most systematic exposition of Paul’s thought, clearly makes justification the center.” Not only in Paul but in the pre-Pauline creedal hymns we find this affirmation (2Ti 1: 9 and Tit 3: 4– 5)...

"Like Schweitzer, a variety of contemporary trends in Pauline studies as well as Reformation scholarship are driven by the presupposition that mystical participation in Christ stands over against a forensic emphasis on Christ’s alien righteousness imputed to believers. 3 Through the interpretive lens of union with Christ we can move beyond the false choice of a legal, judicial, and passive salvation on one hand and a relational, mystical, and transformative participation in Christ on the other. Nevertheless, as I argued in relation to Christ’s atoning work, the integral unity of these motifs is possible only because the latter is grounded in the former. As Geerhardus Vos expressed it,

In our opinion Paul consciously and consistently subordinated the mystical aspect of the relation to Christ to the forensic one. Paul’s mind was to such an extent forensically oriented that he regarded the entire complex of subjective spiritual changes that take place in the believer and of subjective spiritual blessings enjoyed by the believer as the direct outcome of the forensic work of Christ applied in justification. The mystical is based on the forensic, not the forensic on the mystical.

"In Romans 5, the covenantal union of humanity in Adam is contrasted with Christ’s covenantal headship, and then in chapter 6 we encounter his most explicit description of union with Christ in his death and resurrection (Ro 6: 1– 23; cf. 1: 3– 4; 4: 25; 1Co 15: 35– 58). Though they have been “in Christ” in God’s electing grace from all eternity (Eph 1: 4, 11; 2Ti 1: 9), their actual union with Christ occurs in time through the work of the Spirit. Throughout the Pauline corpus we encounter this emphasis on union with Christ."

[bold emphasis added]

Horton, Michael S. (2011-01-04). The Christian Faith: A Systematic Theology for Pilgrims on the Way (Kindle Locations 14544-14565). Zondervan. Kindle Edition.

Thoughts on Sanctification continued...

WSC Q. 35. What is sanctification? Sanctification is the work of God’s free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness.

WCF 13.1 Of Sanctification:  They who are effectually called and regenerated, having a new heart and a new spirit created in them, are further sanctified, really and personally, through the virtue of Christ's death and resurrection, by his Word and Spirit dwelling in them; the dominion of the whole body of sin is destroyed, and the several lusts thereof are more and more weakened and mortified, and they more and more quickened and strengthened, in all saving graces, to the practice of true holiness, without which no man shall see the Lord.

Random thoughts on the above: 
By the indwelling of the Word and Holy Spirit, through the virtue of the benefits of Christ's death and resurrection, believer's are really and personally further sanctified, i.e. renewed to the image of Christ and set apart unto righteousness; the result of regeneration which has created in us a new heart and new spirit (or as the English reformer Thomas Cranmer wrote - a new heart and "new right-will"). The dominion of sin is broken because the body of sin (all our guilt and sins) that stood against us has been nullified by Christ's payment for sin on our behalf. Condemnation and Death because of Sin no longer have any claim on those who are Christ's. By His Word and Spirit we are then enabled to grow in this free saving grace (the benefits of Christ's death and resurrection) and, yes, the domininion of sin is overthrown and its power weakened by Christ's death and resurrection through faith alone in him, his mediation. The "more and more" of WCF 13.1 is qualified by the second and third article of WCF 13:

This sanctification is throughout in the whole man, yet imperfect in this life: there abideth still some remnants of corruption in every part, whence ariseth a continual and irreconcilable war, the flesh lusting against the Spirit, and the Spirit against the flesh... In which war, although the remaining corruption for a time may much prevail.

Christ's death on our behalf, paying our penalty due to our sin, is completely and eternally effectual for us. We died in his death to sin. Therefore we paid the penalty for our sin through Christ, our Surety's death for us. 

Rom. 6:6 - knowing this, that our old man was crucified with him, that the body of sin might be done away, that so we should no longer be in bondage to sin.

The legal body of condemning evidence against the elect is done away! Sin can no longer be held up by the Law before God to condemn us. The Law's charge of guilty rightly due to our sin has been removed as far as the east is from the west through our "death" in Christ on the cross. 

When you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross (Col. 2:13-14).  

When condemnation does creep into my conscience the only safe harbor, then, is not in perfecting my works, but in the Gospel of Christ, the good news encapsulated in Jesus's last words on the cross, "It is finished."

We never (in this life or the next) cease to be the recipients of a fully gracious, God initiated and fully enacted salvation.  

The Bottom Line: Our Father in heaven is not looking for a remedy for the sins of the elect, the impurity of their fallen nature, or the remnant of sin which remains in their every thought, word, and deed other than that wrought by Christ. Hello... Good News... without qualification! 

If God's loving, gracious, and merciful salvation of sinners completed in Christ Jesus needs to be fortified by a purity born of our efforts against sin or pursuing godly works then this Good News of God will soon be replaced by either Despair or Self-Righteousness.

And to aid us in rightly responding to this abundant grace of God we find given to us by the Holy Spirit a new inward and godly desire, born of above, to present ourselves no longer as slaves to sin but slaves to righteousness. And this is why our remaining sin so continues to vex us (Rom. 7 and Gal. 5}! 

So, having then been saved in order that we might be conformed to Christ's image, we're admonished and encouraged by the apostle Paul:

Therefore do not let sin reign in your mortal body so that you obey its lusts, and do not go on presenting the members of your body to sin as instruments of unrighteousness; but present yourselves to God as those alive from the dead, and your members as instruments of righteousness to God. For sin shall not be master over you, for you are not under law but under grace. (Rom. 6:12-14)

Then, as the apostle Paul wrote, let us yield ourselves to the rule of Christ with faith and gratefulness and walk in the path of his righteousness, doing that which is good and acceptable in his sight, not angsting over the weight of our sins nor measuring the merit of our  good works.  Our overbearing load of debt is paid... The righteousness required before God is secured... By Christ alone!

We remain and always will remain the recipients of a fully gracious, God-initiated sanctification and salvation (Eph. 2:8-10)!

Thomas Boston: "Behold Him... Who Has Repaired the Breach..."

"Lastly, Here is a demonstration of the absolute necessity of being united to the Second Adam, who kept the second covenant, and thereby fulfilled the demands of the first covenant. See your absolute need of him; prize him, and flee to him by faith. Behold him with an eye of faith, who has repaired the breach. 

"The first Adam broke the first covenant, by eating of the fruit of the forbidden tree; Christ has repaired the breach, by hanging on a tree, and bearing the curse, for his people. Adam's preposterous love to his wife made him sin: * Christ's love to his spouse made him suffer and satisfy. In a gadrden Adam sinned, and therefore in a garden Christ was buried. Eating ruined man, and by eating he is saved again. By eating the forbidden fruit all died; and by eating Christ's flesh and drinking his blood by faith, the soul gets life again, John vi. 57. 

"O then have recourse to Christ; and thus shall you be saved from the ruins of the fall, and have an interest in the covenant made with Christ, the condition of which being already fulfilled by him, can never be broken, or they who are once in it ever fall out of it again."

* Whether Adam sinned out of love to his wife, is a question that has been agitated  among divines. A satisfactory solution of it may be seen in Witsius*s Economy, 8cc. book i. chap. viii. - 9.

Thomas Boston. A View of the Covenant of Works, p. 88 

Believer's Obedience Resides Under the Mercy Seat...

Were one to have stood in the Holy of Holies and looked upon the Ark of the Covenant, one would have seen that the Mercy seat sits above, covering the Ark in which sit the Ten Commandments of the Law. This divine design is a type (Lev. 16:14) that points to several New Covenant realities of Christ's finished work. One of which is that the good works of those justified by Christ's blood, though still stained with the remnant of sin as measured by God's holy requirement of the Law, are yet accepted fully and mercifully by God for the alone sake of Jesus Christ's perfect obedience and atonement imputed to them.

Regarding our works of obedience to God and Christ the Mercy Seat, Westminster Confession of Faith 16.6 explains the anti-type...

Notwithstanding, the persons of believers being accepted through Christ, their good works also are accepted in him; not as though they were in this life wholly unblamable and unreprovable in God's sight; but that he, looking upon them in his Son, is pleased to accept and reward that which is sincere, although accompanied with many weaknesses and imperfections.

Calvin: To Admit Sinner into Union with God is to Justify Him...

"Hence the Apostle shows that man is at enmity with God until he is restored to favor by Christ, (Romans 5:8- l 0.) When the Lord, therefore, admits him to union, he is said to justify him, because he can neither receive him into favor, nor unite him to himself, without changing his condition from that of a sinner into that of a righteous man. He adds that this is done by remission of sins. For if those whom the Lord has reconciled to himself are estimated by works, they will still prove to be in reality sinners, while they ought to be pure and free from sin. It is evident therefore, that the only way in which those whom God embraces are made righteous, is by having their pollutions wiped away by the remission of sins, so that this justification may be termed in one word the remission of sins." [emphasis added]

Calvin, John. Institutes: Christian Religion Book 3.9.22

Calvin: Gift of Adoption by which God Admits Us into a Union with Christ...

"though we may be pressed down by adversity, yet we are not excluded from the number of God's children, since we see him going before us who was by nature his only Son; for that we are counted his children is owing only to the gift of adoption by which he admits us into a union with him, who alone lays claim to this honor in his own right." [emphasis added]

Calvin, John. Complete Commentaries, Hebrews 5:7

Calvin: Until Imputation of Righteousness "Union with God Cannot Be Hoped For"

"We yesterday compared this passage of Habakkuk with the interpretation of Paul, who draws this inference, that we are justified by faith without the works of the law, because the Prophet teaches us that we are to live by faith, for the way of life and of righteousness is the same, inasmuch as life is not to be otherwise sought by us than through the paternal favor of God. This then is our life--to be united to God; but this union with God cannot be hoped for by us while he imputes sins to us; for as he is just and cannot deny himself, iniquity must be ever hated by him. Then as long as he regards us as sinners, he must necessarily hold us as hateful to him. Where the hatred of God is, there is death and ruin. It then follows, that we can have no hope of life until we be reconciled to God, and there is no other way by which God can restore us to favor, but by regarding and counting us as just. It hence follows, that Paul reasons correctly, when he leads us from life to righteousness; for they are two things which are connected and inseparable." [emphasis added]

Calvin, John. Complete Commentaries - Habakkuk.

Calvin: Justified, Believer Becomes One with Christ...

"To sum up the whole, this passage, first, teaches us to behold Christ with the eyes of faith; and, secondly, it informs us, that every one who is regenerated by the Spirit, and gives himself up entirely to God for true justification, is thus admitted to the closest union with Christ, and becomes one with him."

Calvin, John. Complete Commentaries - Matthew 12:48