A.T. Robertson Commentary Romans 4:11

"and he received the sign of circumcision, a seal of the righteousness of the faith which he had while he was in uncircumcision; that he might be the father of all them that believe, though they be in uncircumcision, that righteousness might be reckoned unto them;" — Romans 4:11 (ASV)

The sign of circumcision (σημειον περιτομης). It is the genitive of apposition, circumcision being the sign.

A seal of the righteousness of the faith (σφραγιδα της δικαιοσυνης της πιστεως). Σφραγις is old word for the seal placed on books (Revelation 5:1), for a signet-ring (Revelation 7:2), the stamp made by the seal (2 Timothy 2:19), that by which anything is confirmed (1 Corinthians 9:2) as here. The circumcision did not convey the righteousness, but only gave outward confirmation. It came by faith and "the faith which he had while in uncircumcision" (της εν τη ακροβυστια), "the in the state of uncircumcision faith." Whatever parallel exists between baptism and circumcision as here stated by Paul argues for faith before baptism and for baptism as the sign and seal of the faith already had before baptism.

That he might be (εις το εινα αυτον). This idiom may be God's purpose (contemplated result) as in εις το λογισθηνα below, or even actual result (so that he was) as in 1:20.

Though they be in uncircumcision (δι' ακροβυστιας). Simply, "of those who believe while in the condition of uncircumcision."