Here is a quote on Baptism from Steve Wellum. Wellum co-edited a new book Believer’s Baptism, and has been interviewed by Justin Taylor on the subject.The root of the issue:
Now, let’s be clear. Wellum’s chapter in the book may make some baptists a little sqeemish, but please, do not be alarmed. I think he does lay a biblical, and historically baptistic, understanding of baptism and its relationship to the Old and New Covenants. The above mentioned book is a very important work to be coming from baptists right now, so do take up and read. Also, if you want an even more thorough treatment on the subject, Fred Malone’s The Baptism of Disciples Alone is a must read.
Under the old covenant, both “seeds” (physical and spiritual) received the covenant sign of circumcision and both were viewed as full covenant members in the national sense, even though it was only the remnant who were the true spiritual seed of Abraham. But this kind of distinction is not legitimate under the new covenant where the locus of the covenant community and the elect are the same. In other words, one cannot speak of a “remnant” in the new covenant community, like one could under the old covenant. All those who are “in Christ” are a regenerate people, and as such it is only they who may receive the sign of the covenant, namely baptism.