Romans 6:1-6
1 What shall we say then? Shall we continue in sin, that grace may abound?
2 God forbid. How shall we, that are dead to sin, live any longer therein?
3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
4 Therefore we are buried with him by 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.
5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Abridged Commentary - Romans 6:3-4
6:3-4 In Romans 6:3-4, Galatians 3:27, and Colossians 2:12, the Apostle Paul speaks of being "baptized into Jesus Christ", and being "buried with (Jesus Christ) in baptism". In each case, it seems that this is either a reference to the baptism of the Spirit into the Body of Christ, mentioned in 1 Corinthians 12:12-13, which happens when a person believes in Jesus (Ephesians 1:12-14), or, it is the Apostle Paul expounding the symbolism of water baptism, which represents dying to oneself (Luke 9:23, Galatians 2:20), and walking in the newness of life secured by the death and resurrection of Jesus Christ.
The Grace New Testament Commentary - Romans 6:3-4
6:3 Many interpreters see water baptism here. But we know that baptism by the Holy Spirit was a doctrine profoundly significant to Paul. It is what forms the Body of Christ (1 Cor 12:13). Thus our spiritual union with Jesus Christ is affected by the Holy Spirit's baptism, and it is precisely to our union with Him that the following verses in Romans appeal.
Moreover, in no NT passage is water baptism unambiguously referred to as baptism into Christ Jesus (baptism into the Body of Christ Jesus). Where water baptism is linked explicitly to Jesus Christ, it is always in His name (Acts 2:38; 8:16; 10:48; 19:5: 1 Cor 1:13, 15 [by inference]). The baptism of the Holy Spirit means that believers have been baptized into His death. This union with Christ in His death is in fact the key to a new life experience.
6:4 Our spiritual union involves "immersion" into Christ's death. Immersion into His death effectively "purifies" the inner man from sin, rendering him dead to it (see vv 5-11; also Titus 3:4-7). The Christian has been united with Christ in His death so that he may also share in His resurrected life (walk in newness of life).
As Paul will go on to point out, especially in Romans 8:1-13, this glorious resurrection power is precisely what will enable the believer to surmount the impediment of his sinful body. The issue before Paul's mind, both here and up to 8:13, is how those who are alive from the dead can live like it. In other words, how can such people walk in newness of life?