A few years ago I was seriously looking at ministry in the Church of England and spend time studying the Anglican/Presbyterian position on baptism. Should be said that by that point I'd been christened and then confirmed in my early teens, before becoming a Christian at 18 and being baptised at 19. Bishop Wallace Benn conmmended John Murray's Christian Baptism to me as the book that convinced him on the issue. I read it and I wasn't. I'd not particularly found any books that argued well the other way - most baptists seem to just say 'it's obvious', which is reasonable enough but not all that helpful!
This seems to get there. PDF: Stephen J. Wellum on Baptism and the Relationship between the Covenants from Believers Baptism
ht: JT - 'baptism as a test case for biblical theology'
"...circumcision, as a type, pointed to a spiritual regeneration. Baptism, on the other hand, testifies that by faith these realities have occurred. Baptism marks and defines the children of God, those who believe in Messiah Jesus. That is why we baptize only those who have confessed Jesus as Lord, who have experienced his power, who are, by faith and spiritual rebirth, Abraham’s true spiritual seed." (PDF p63 of 66)Wellum continues...
"What does baptism signify? As already stated, it signifies a believer’s union with Christ, by grace through faith, and all the benefits that result from that union. It testifi es that one has enteredinto the realities of the new covenant and as such, has experienced regeneration, the gift and down-payment of the Spirit, and the forgiveness of sin. It graphically signifies that a believer is now a member of the body of Christ..." (PDF p64 of 66)We meet and sing the gospel. We meet and preach the gospel. We eat communion and preach the gospel. We baptise and preach the gospel. It's simple really (!), gospel, gospel, gospel because "Christ has come, God deals with all nations directly through his Son".