Grace Alone through Faith Alone in Christ Alone: God's Justification and Sanctification Intersection

“And you shall put the mercy seat on the top of the ark, and in the ark you shall put the testimony that I shall give you. There I will meet with you, and from above the mercy seat, from between the two cherubim that are on the ark of the testimony, I will speak with you about all that I will give you in commandment for the people of Israel.” Exodus 25:21-22

The Mercy Seat sits above the Ark of the Covenant of the Old Testament in which is contained the Ten Commandments of the Law. This is a type pointing to Jesus who on the cross bore the penalty of our sins and fulfilled the righteous demand of that Law. It is there that God meets with his people.

Our imperfect obedience (i.e. believers' obedience in of itself is still measured as sinful before God's holy and unyielding standard) does not attain to the righteouseness of the Law. Nonetheless it is accepted by God our Father for Christ's sake alone, the Mercy Seat who covers our sins and our less-than-righteous good works. Regarding those imperfect works of believers the Westminster Confession of Faith states -

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. WCF 16:6 

Through faith alone we are united with Christ. Now being identified with him through faith, not only our flawed persons are accepted and rewarded but also our flawed works. 

Grace alone through faith alone in Christ alone - God's justification and sanctification formula!