"And he received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised" (Romans 4:11a)
This is an interesting verse, in that this explains what I understand baptism to be, as a credobaptist: "the sign... [and] seal" of the righteousness one has by faith.
Though Paul does not mention baptism here, I think from other places it is clear that there is continuity in this aspect between circumcision in the Old Covenant and baptism in the New Covenant. This is what Presbyterians believe, but obviously to an extent that no credobaptist, by definition, can agree with.
Obviously an infant cannot have this "righteousness...by faith", as they cannot exhibit faith in Jesus, given their undeveloped minds. But that makes this verse somewhat difficult, because Abraham gave this "sign" (circumcision) to his children right after giving it to himself (cf. Genesis 17). Did it mean for his children what it meant for him (according to Paul in Romans 4), or not?
(It is key to note that Paul doesn't say it was the sign and seal of faith for Abraham's children too, and rightly so, given the aforementioned fact that they could have no such faith until a later age—and perhaps never, as was the case for many of Abraham's lineage, a la Romans 9:6.)
Paul was fully aware of the problem such a proposition—infants having saving faith—would create, and it seems that it is even besides the point. In Romans 4, it seems Paul is purposely giving the New Covenant understanding of covenantal entrance with a definition that precludes the Old Covenant's inclusion of infants, in order to illustrate this change as it has providentially occurred in the institution of (credo)baptism.
Thus, Paul, under the inspiration of the Spirit, saw the fulfilled meaning of circumcision—which was originally the ongoing visible marker of covenant identity—as the one-time sign and seal of the faith one already possesses.