A minister of the circumcision (diakonon peritomēs). Objective genitive, “a minister to the circumcision.” Diakonon is predicate accusative with gegenēsthai (perfect passive infinitive of ginomai in indirect assertion after legō, I say) and in apposition with Christon, accusative of general reference with the infinitive. See Gal 4:4.
That he might confirm (eis to bebaiōsai). Purpose clause with eis to and the infinitive bebaiōsai (first aorist active of bebaioō, to make stand).
The promises given unto the fathers (tas epaggelias tōn paterōn). No “given” in the Greek, just the objective genitive, “the promises to the fathers.” See note on Rom 9:4, Rom 9:5.