Charles Ellicott 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)

Now I say...—Rather, For I say. My doctrine is that Christ came with a twofold purpose: on the one hand, with a mission to the Jews, the chosen circumcised race, to vindicate to them the truthfulness of God in respect to His promises, by Himself confirming and fulfilling those promises; and, on the other hand, to exhibit the mercy of God in rescuing the Gentiles from their state of condemnation, and giving them cause to glorify God’s name.

Was...—This is the reading of the Vatican MS. and Paris rescript; the Sinaitic and Alexandrine have, hath been made.

For the truth of God—i.e., to make good the truthfulness of God in keeping His promises.