The act of baptism is a re-enactment of the death, burial and resurrection of Jesus.  No matter at what age we were baptized, we are called to live out the reality of our baptism.  The Roman Christians were baptized but may have thought that they could continue in sin to get more grace.  What Paul told them was not to get “re-baptized”, but to live out the meaning of their baptism.  It means to be dead to sin as a willful act and to be free from slavery to sin (Rom 6-8).  To live out our baptism is to walk in new resurrected life because the Spirit of the Risen One lives within us.  It can be done, only by the indwelling Holy Spirit.

“What shall we say then? Are we to continue in sin that grace may abound?  By no means! How can we who died to sin still live in it?  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.” Romans 6:1-4 (ESV)