Matthew 17:6

Matthew names Abraham because Abraham was the first type of the two peoples who would believe in Christ. He believed while still uncircumcised, and after circumcision he remained faithful.

Matthew names David especially because David too was a type of Christ. When Saul was cast out as rejected, David was appointed second as a king well-pleasing to God. In the same way, when the first human — Adam — was cast out through disobedience and no longer ruled the rest, a second Adam was brought in: Christ, king of all, so that what was lost in Adam might be recovered for humankind through Christ. He is king not only of earth but of heaven. He is called Son of Man by nature because he has all that Adam had, while now being God.

Abraham believed God while uncircumcised; then, after receiving the promises and being circumcised, he kept the faith unharmed. So the verse rightly rises to another contemplation: the church of Christ from circumcision and from the nations. Abraham was a type of those two peoples who have believed in Christ.

There uncircumcision came before circumcision. Uncircumcision is nature’s work; circumcision is command and law. That is why the former led, and afterward circumcision turns back into it — because nature together with law is better than the two standing apart.

David too was a type of Christ. When Saul was cast out, David was brought in instead. So also, when Adam was driven from Eden through disobedience, the second Adam was brought in instead, that he might restore the first Adam to the ancient order.