I do NOT believe in baptismal regeneration, just to be clear. I use the word "Christian" to describe anyone who has been biblically baptized, just as the circumcised were called "Jews," yet not all were regenerate.

Baptism "replaces" circumcision just as the Lord's Day "replaces" the seventh-day Sabbath. Those who observe them are called "Jews" or "Christians" merely by association.

I hope that's clearer.