5 Wherefore also Mark, the interpreter and follower of Peter, does thus commence his Gospel narrative: “The beginning of the Gospel of Jesus Christ, the Son of God; as it is written in the prophets, Behold, I send My messenger before Your face, which shall prepare Your way. The voice of one crying in the wilderness, Prepare the way of the Lord, make the paths straight before our God.” Plainly does the commencement of the Gospel quote the words of the holy prophets, and point out Him at once, whom they confessed as God and Lord; Him, the Father of our Lord Jesus Christ, who had also made promise to Him, that He would send His messenger before His face, who was John, crying in the wilderness, in “the spirit and power of Elias,” “Prepare the way of the Lord, make straight paths before our God.”
For the prophets did not announce one and another God, but one and the same; under various aspects, however, and many titles. For varied and rich in attribute is the Father, as I have already shown in the book preceding this; and I shall show [the same truth] from the prophets themselves in the further course of this work. Also, towards the conclusion of his Gospel, Mark says: “So then, after the Lord Jesus had spoken to them, He was received up into heaven, and sits on the right hand of God;” confirming what had been spoken by the prophet: “The Lord said to my Lord, Sit on My right hand, until I make Your foes Your footstool.” Thus God and the Father are truly one and the same; He who was announced by the prophets, and handed down by the true Gospel; whom we Christians worship and love with the whole heart, as the Maker of heaven and earth, and of all things therein.
Source: Adversus haereses (New Advent)