Colossians 2:12 “having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead. “
Second, baptism “expresses union with Christ in His death and resurrection.” The clearest teaching on this is Romans 6:3-4.
Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
In the wider context of Romans, I think it would be a mistake to say that water-baptism is the means of our being united to Christ. In Romans faith is the means by which we are united to Christ and justified. But we show this faith—we say this faith and signify this faith and symbolize this faith—with the act of baptism. Faith unites to Christ; baptism symbolizes the union.
An analogy would be saying, “With this ring I thee wed.” When we say that we don’t mean that the ring or the putting of the ring on the finger is what makes us married. No, it shows the covenant and symbolizes the covenant, but the covenant-making vows make the marriage. So it is with faith and baptism.
So similarly Paul is saying, “With this baptism you are united to Christ.” And the point we are focusing on here is that we are united to him in his death and burial and resurrection. “We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.” So the imagery of baptism is death, burial, and resurrection. Christ was buried and raised to new life.
In baptism, by faith, we are united with Christ in his death, burial, and resurrection. Baptism dramatically portrays what happened spiritually when you received Christ: Your old self of unbelief and rebellion and idolatry died, and a new you of faith and submission and treasuring Christ came into being. That’s what you confess to the world and to heaven when you are baptized.