A.T. Robertson Commentary Romans 15:8

"For I say that Christ hath been made a minister of the circumcision for the truth of God, that he might confirm the promises [given] unto the fathers," — Romans 15:8 (ASV)

A minister of the circumcision (διακονον περιτομης). Objective genitive, "a minister to the circumcision." Διακονον is predicate accusative with γεγενησθα (perfect passive infinitive of γινομα in indirect assertion after λεγω, I say) and in apposition with Χριστον, accusative of general reference with the infinitive. See Ga 4:4f.

That he might confirm (εις το βεβαιωσα). Purpose clause with εις το and the infinitive βεβαιωσα (first aorist active of βεβαιοω, to make stand).

The promises given unto the fathers (τας επαγγελιας των πατερων). No "given" in the Greek, just the objective genitive, "the promises to the fathers." See 9:4,5.