7 From the words of Peter, therefore, which he addressed in Cæsarea to Cornelius the centurion, and those Gentiles with him, to whom the word of God was first preached, we can understand what the apostles used to preach, the nature of their preaching, and their idea with regard to God. For this Cornelius was, it is said, “a devout man, and one who feared God with all his house, giving much alms to the people, and praying to God always. He saw therefore, about the ninth hour of the day, an angel of God coming in to him, and saying, Your alms have come up for a memorial before God.
Wherefore send to Simon, who is called Peter.” But when Peter saw the vision, in which the voice from heaven said to him, “What God has cleansed, call not common,” this happened [to teach him] that the God who had, through the law, distinguished between clean and unclean, was He who had purified the Gentiles through the blood of His Son— He whom also Cornelius worshipped; to whom Peter, coming in, said, “Of a truth I perceive that God is no respecter of persons: but in every nation, he that fears Him, and works righteousness, is acceptable to Him.” He thus clearly indicates, that He whom Cornelius had previously feared as God, of whom he had heard through the law and the prophets, for whose sake also he used to give alms, is, in truth, God.
The knowledge of the Son was, however, wanting to him; therefore did [Peter] add, “The word, you know, which was published throughout all Judea, beginning from Galilee, after the baptism which John preached, Jesus of Nazareth, how God anointed Him with the Holy Ghost, and with power; who went about doing good, and healing all that were oppressed of the devil; for God was with Him. And we are witnesses of all those things which He did both in the land of the Jews and in Jerusalem; whom they slew, hanging Him on a beam of wood: Him God raised up the third day, and showed Him openly; not to all the people, but unto us, witnesses chosen before of God, who ate and drank with Him after the resurrection from the dead.
And He commanded us to preach unto the people, and to testify that it is He which was ordained of God to be the Judge of quick and dead. To Him give all the prophets witness, that, through His name, every one that believes in Him does receive remission of sins.” The apostles, therefore, did preach the Son of God, of whom men were ignorant; and His advent, to those who had been already instructed as to God; but they did not bring in another god. For if Peter had known any such thing, he would have preached freely to the Gentiles, that the God of the Jews was indeed one, but the God of the Christians another; and all of them, doubtless, being awe-struck because of the vision of the angel, would have believed whatever he told them.
But it is evident from Peter's words that he did indeed still retain the God who was already known to them; but he also bore witness to them that Jesus Christ was the Son of God, the Judge of quick and dead, into whom he did also command them to be baptized for the remission of sins; and not this alone, but he witnessed that Jesus was Himself the Son of God, who also, having been anointed with the Holy Spirit, is called Jesus Christ. And He is the same being that was born of Mary, as the testimony of Peter implies.
Can it really be, that Peter was not at that time as yet in possession of the perfect knowledge which these men discovered afterwards? According to them, therefore, Peter was imperfect, and the rest of the apostles were imperfect; and so it would be fitting that they, coming to life again, should become disciples of these men, in order that they too might be made perfect. But this is truly ridiculous. These men, in fact, are proved to be not disciples of the apostles, but of their own wicked notions.
To this cause also are due the various opinions which exist among them, inasmuch as each one adopted error just as he was capable [of embracing it]. But the Church throughout all the world, having its origin firm from the apostles, perseveres in one and the same opinion with regard to God and His Son.
Source: Adversus haereses (New Advent)