Helps in Understanding Baptism in Romans Chapter Six

To understand Paul's use of the word baptism involves not only understanding covenantally what is meant by 'union.', but also the significant difference between a sacrament as a sign - that which signifies a truth that itself is not - and the reality that the sign itself represents and points to. In a word, the bread and wine of the Lord's Supper are not the actual body and blood of our Lord Jesus Christ but signify them. And the water of baptism is not that which actually cleanses from sin nor is it the agent of spiritual union with Christ but represents those blessings bestowed by Christ himself through the Spirit on his elect in the office of baptizer of his people (Matthew 3:11). Speaking of the sacraments, Augustine wrote, "For they be signs of things, being one thing, and signifying another."

Robert Haldane's Romans Commentary, Chapter 6 -

Ver. 3. — Know ye not, that so many of was were baptized into Jesus Christ, were baptized into His death? 

In the verse before us, the Apostle proves that Christians are dead to sin, because they died with Christ. The rite of baptism exhibits Christians as dying, as buried, and as risen with Christ. Know ye not. — He refers to what he is now declaring as a thing well known to those whom he addresses. Baptized into Jesus Christ. — By faith believers are made one with Christ: they become members of His body. This oneness is represented emblematically by baptism. Baptized into His death. — In baptism, they are also represented as dying with Christ. This rite, then, proceeds on the fact that they have died with Him who bore their sins. Thus the satisfaction rendered to the justice of God by Him, is a satisfaction from them, as they are constituent parts of His body. The believer is one with Christ as truly as he was one with Adam — he dies with Christ as truly as he died with Adam. Christ’s righteousness is his as truly as Adam’s sin was his. By a Divine constitution, all Adam’s posterity are one with him, and so his first sin is really and truly theirs. By a similar Divine constitution, all Christ’s people are one with Him, and His obedience is as truly theirs as if they had yielded it, and His death as if they had suffered it. When it is said that Christians have died with Christ, there is no more figure than when it is said that they have died in Adam. 

The figure of baptism was very early mistaken for a reality, and accordingly some of the fathers speak of the baptized person as truly born again in the water. They supposed him to go into the water with all his sins upon him, and to come out of it without them. This indeed is the case with baptism figuratively. But the carnal mind soon turned the figure into a reality. It appears to the impatience of man too tedious and ineffectual a way to wait on God’s method of converting sinners by His Holy Spirit through the truth, and therefore they have effected this much more extensively by the performance of external rites. When, according to many, the rite is observed, it cannot be doubted that the truth denoted by it has been accomplished. The same disposition has been the origin of Transubstantiation. The bread and wine in the Lord’s Supper are figuratively the body and blood of Christ; but they have been turned into the real body, blood, soul, and divinity of the Lord, and the external rite has become salvation. 

So many of us. — This does not imply that any of those to whom the Apostle wrote were not baptized, for there could be no room for such a possibility. It applies to the whole of them, as well as to himself, and not merely to a part. It amounts to the same thing as if it had been said, ‘We who were baptized;’ as in <440324>Acts 3:24, ‘As many as have spoken,’ that is, all who have spoken, for all the Prophets spoke.  

Ver. 4. — Therefore we are buried with him baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. 

The death of Christ was the means by which sin was destroyed, and His burial the proof of the reality of His death. Christians are therefore represented as buried with Him by baptism into His death, in token that they really died with Him; and if buried with Him, it is not that they shall remain in the grave, but that, as Christ arose from the dead, they should also rise. Their baptism, then, is the figure of their complete deliverance from the guilt of sin, signifying that God places to their account the death of Christ as their own death: it is also a figure of their purification and resurrection for the service of God. 

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Michael Horton's Systematic Theology, The Christian Fatih.

This means that union with Christ is a soteriological category. However true it may be that all creatures exist in analogical dependence on God’s being, the Spirit communicates Christ’s eschatological righteousness and life, not the divine essence, to believers. Paul explains in Romans 5 that we enter the world united to Adam as our covenant head, with his guilt imputed and corruption imparted. From the womb, we are declared ungodly, and we live out that status in daily unbelief and sinful actions. Baptized into Christ, we are transferred to another covenant head, who is the source of righteousness imputed and holiness imparted. Hence, Calvin encourages us to find our purity in Christ’s virginal conception, our anointing with the Spirit in his baptism, our mortification in his tomb, our life in his resurrection, and the gifts of the Spirit in his sending of the Spirit at Pentecost, which is echoed also in the Great Litany of the Book of Common Prayer. 

Therefore, union with Christ is to be understood in covenantal terms. The Messiah not only saves; he is the corporate head of the people whom he represents and makes to share in the spoils of his victory. As goes the King, so goes the kingdom. As the firstfruits of the whole harvest, Jesus Christ is not merely an example to be imitated by his followers, but the head of a covenantal body to be incorporated into by the Spirit. Whatever is true concerning the King must also be true in principle concerning his people. 46 This is what it means to be baptized into Christ... 

Similarly, in 1 Corinthians 10 and 11 Paul teaches that his readers were baptized into Christ just as their old covenant predecessors were “baptized into Moses in the cloud and in the sea” (1Co 10: 2). However, the exodus generation enters the true promised land only together with us, their entry having been foreshadowed when they “all ate the same spiritual food, and all drank the same spiritual drink.” “For they drank from the spiritual Rock that followed them, and the Rock was Christ” (vv. 3–4)...

The Scriptures locate believers “in Christ,” which means in his church: “As many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus” (Gal 3: 27—28).

The Priority of Justifying Faith - Owen

 "Wherefore we say the faith by which we are justified is such as is not found in any but those who are made partakers of the Holy Ghost, and by him united to Christ - whose nature is renewed, and in whom there is a principle of all grace and purpose of obedience. Only we say that it is not any other grace, nor any obedience that gives life and form unto this faith; but it is this faith that gives life and efficacy to all other graces, and form to all evangelical obedience; all gospel holiness and good works presupposes faith as their root and principle: and without which there are no such things. Yet we do not assign to them the same influence to our justification which faith hath, nor indeed any influence whatever. We are justified by faith alone. For no other grace is capable of the office of faith in justification nor can be joined with it, to receive Christ and the promise of life by him, and to give glory to God on their account."

John Owen: The Doctrine of Justification by Faith Through the Imputation of the Righteousness of Christ - Explained, Confirmed, and Defended,  p. 60

[emphasis added]