Adam Clarke on Galatians 6:15

In Christ Jesus - Under the dispensation of the Gospel, of which he is head and supreme, neither circumcision - nothing that the Jew can boast of, nothing that the Gentile can call excellent, availeth any thing - can in the least contribute to the salvation of the soul.

But a new creature - Αλλα καινη κτισις· But a new creation; not a new creature merely, (for this might be restrained to any new power or faculty), but a total renewal of the whole man, of all the powers and passions of the soul; and as creation could not be effected but by the power of the Almighty, so this change cannot be effected but by the same energy; no circumcision can do this; only the power that made the man at first can new make him. See the note on 1Cor 7:19, and on 2Cor 5:17 (note).