Do you not know -- what happened to you in your baptism?
Without a doubt all of Paul’s Christian readers would have remembered the time and event of their immersion, since this was a part of the basic presentation of the gospel and of becoming a Christian. However, they may not have understood the deeper spiritual significance of this act; this is what Paul now explains. - CPNIV
as many [all] of us -- All who were baptized and professed to be Christians. As this renunciation of sin had been thus made by all who have been baptized into Christ, so this objection could not have reference to us Christians in any manner. (BN)
were baptized -- Baptism denotes dedication to the service to him in whose name we are baptized. One of its design is to show we renounce sin (repentance Acts 2:38; Luke 13:3;)
... in the NT there is no such thing as an unbaptized Christian. - CPNIV
baptism -- Baptism in the early church showed the new believer’s public profession of faith and committment to Christ as Lord and one’s readiness to live (obey) Christ in his manner of life. The earliest baptismal formula, to be repeated by the candidate, was "I believe Jesus is Lord" (cf. Romans 10:9-13). This public declaration was a formal act that showed one’s repentance and readiness to be baptized (Acts 2:38; Acts 8:37). In one’s conversion to Christ it was not optional. Jesus commanded it (cf. Matthew 28:19-20), and exemplified it, (cf. Matthew 3:13; Mark 1:9; Luke 3:21) and it became part of the Apostolic sermons and practices as seen in the book of Acts.
EXAMPLES OF CONVERSION:
1. Pentecost- Acts 2:22 & Acts 2:36-47
4. Paul (Saul)- Acts 9:1-20 & Acts 22:6-16
5. Cornelius– Acts 10:1-48 & Acts 11:1-18
7. Philippian jailer- Acts 16:23-34
into -- εἰς eis. This is the word which is used in Matthew 28:19, “Teach all nations, baptizing them into [εἰς eis] the name of the Father, and of the son, and of the Holy Spirit."
It means one enters into a new relationship, a union with another.
To be baptized into Christ means to be baptized for the purpose of entering into a specific relationship with him, or into a living union with him. As Moo says (I:377), the preposition “into” (eis) has the connotation of movement from one space to another, as well as the connotation of purpose. Thus as Moo puts it, “baptized into Christ” means “baptized with a view to being united with Christ.” See Galatians 3:27. - CPNIV
were baptized into His death -- Baptism is symbolically the burial of the old man of sin [Romans 6:6; Colossians 3:9; ]; and in the likeness of Jesus’ death where he shed His blood, his blood cleanses us of sin [1 John 1:7; Revelation 1:5 ] and we receive forgiveness [Ephesians 1:7; Colossians 1:14; Acts 2:38 ] to be raised from death to a new life in the Father, the Son, and the Holy Spirit [Rom 6.5;
Baptism is a "form" of the doctrine of the death, burial, and resurrection of Christ (1 Corinthians 15:1-4; Romans 6:17) which we are to obey in being delivered for sin.
The argument is that a burial implies death. Baptism is a burial, therefore its subject has died. As Christ died for our sin, we die to living the way of sin; as the Crucified Christ was buried, we who have died to living the sinful way of life are buried with him. As death and burial separate one from the natural life, so death to sin and burial into Christ should completely sever our relation to living a life serving sin. - PNT