A.T. Robertson Commentary Romans 4:12

"and the father of circumcision to them who not only are of the circumcision, but who also walk in the steps of that faith of our father Abraham which he had in uncircumcision." — Romans 4:12 (ASV)

The father of circumcision (πατερα περιτομης). The accusative with εις το εινα to be repeated from verse 11. Lightfoot takes it to mean, not "a father of a circumcised progeny," but "a father belonging to circumcision," a less natural interpretation.

But who also walk (αλλα κα τοις στοιχουσιν). The use of τοις here is hard to explain, for ου μονον and αλλα κα both come after the preceding τοις. All the MSS. have it thus. A primitive error in a copyist is suggested by Hort who would omit the second τοις. Lightfoot regards it less seriously and would repeat the second τοις in the English: "To those who are, I do not say of circumcision only, but also to those who walk."

In the steps (τοις ιχνεσιν). Locative case. See on 2 Corinthians 12:18. Στοιχεω is military term, to walk in file as in Ga 5:25; Php 3:16.