Paul, when writing to the Romans, has explained this very point: "Paul, an apostle of Jesus Christ, predestinated unto the Gospel of God, which He had promised by His prophets in the holy Scriptures, concerning His Son, who was made to Him of the seed of David according to the flesh, who was predestinated the Son of God with power through the Spirit of holiness, by the resurrection from the dead of our Lord Jesus Christ." And again, writing to the Romans about Israel, he says: "Whose are the fathers, and from whom is Christ according to the flesh, who is God over all, blessed for ever." And again, in his Epistle to the Galatians, he says: "But when the fulness of time had come, God sent forth His Son, made of a woman, made under the law, to redeem them that were under the law, that we might receive the adoption;" plainly indicating one God, who did by the prophets make promise of the Son, and one Jesus Christ our Lord, who was of the seed of David according to His birth from Mary; and that Jesus Christ was appointed the Son of God with power, according to the Spirit of holiness, by the resurrection from the dead, as being the first begotten in all the creation; the Son of God being made the Son Of man, that through Him we may receive the adoption,-humanity sustaining, and receiving, and embracing the Son of God.
Source: Irenaeus Against Heresies Book 3