Which I didn't to have! But there we are, always good to keep playing and thinking about stuff and so we continue with Paul's word's to the Colossians as found in chapter two:
'In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of Christ, 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.'
Brian Chapell1 leads me into a place whereby Paul's words are thought of as acting as a reminder that salvation comes through faith, and that the rite of circumcision that once signified the benefits of Abraham’s covenant has been replaced by baptism. A thought that finds some support in Calvin's Institutes as it lead me to a place where the fulfilment and truth of baptism are likewise the truth and fulfilment of circumcision too - signifying one and the same thing (this is what notes from various lectures tell me).
One of the biggest concerns I have when trying to see the baby for the bathwater is that Paul's teaching is more often than not a remedy against error and heresy rather than him trying to establish a theology of whatever is before us - in this case baptism. Different tacks are taken when correction rather that establishment are the key and these, whilst 'fit for role' are often confusing for parctice :-)
In the Romans two (28-29) passage I understand the 'circumcision of the heart' to be a mark of the believer, whether circumcised in the flesh or otherwise, and more accurately of one who is not merely a follower of 'The Way' but is also a disciple. This would fit in with other passages (old and new) that talk of the uncircumcision and the way that uncircumcised who keep the laws have the 'circumcision of the heart' and put those who are circumcised in the flesh, who do not, to shame.
It does therefore talk of Jew and Gentile who have this inner circumcision - where the place of baptism is makes for interesting conjecture.
The Romans six 'baptism into Christ Jesus' and the reality that baptism finds its fulfilment in the cross makes the 'buried with him' all the more real as we pass below the surface of the water (a reason I prefer immersion to sprinkling) and are buried beneath the waves. That said, NT practice was to do that with the sprogs rather than flick water at them. We are baptised into the merits and benefits of Christ (and His salvific and atoning act) in exactly the same way that the covenant of circumcision brings the Jews into the benefits of Abraham.
I can understand your desire for separation and wonder if this owes as much to your baptist stance as the seeking interchangeability does for those who are of the paedobaptism persuasion?
Here's a bit more grist to the mill either way (I'll do arguments against and sacramental stuff shortly)
Hope this helps
1 A Pastoral Overview of Infant Baptism, Gregg Strawbridge; 2003, Presbyterian and Reformed
ISBN 10: 0875525547