1 With regard to those (the Marcionites) who allege that Paul alone knew the truth, and that to him the mystery was manifested by revelation, let Paul himself convict them, when he says, that one and the same God wrought in Peter for the apostolate of the circumcision, and in himself for the Gentiles. Peter, therefore, was an apostle of that very God whose was also Paul; and Him whom Peter preached as God among those of the circumcision, and likewise the Son of God, did Paul [declare] also among the Gentiles. For our Lord never came to save Paul alone, nor is God so limited in means, that He should have but one apostle who knew the dispensation of His Son. And again, when Paul says, “How beautiful are the feet of those bringing glad tidings of good things, and preaching the Gospel of peace,” he shows clearly that it was not merely one, but there were many who used to preach the truth. And again, in the Epistle to the Corinthians, when he had recounted all those who had seen God after the resurrection, he says in continuation, “But whether it were I or they, so we preach, and so you believed,” acknowledging as one and the same, the preaching of all those who saw God after the resurrection from the dead.
2. And again, the Lord replied to Philip, who wished to behold the Father, “Have I been so long a time with you, and yet you have not known Me, Philip? He that sees Me, sees also the Father; and how do you say then, Show us the Father? For I am in the Father, and the Father in Me; and henceforth you know Him, and have seen Him.” To these men, therefore, did the Lord bear witness, that in Himself they had both known and seen the Father (and the Father is truth). To allege, then, that these men did not know the truth, is to act the part of false witnesses, and of those who have been alienated from the doctrine of Christ. For why did the Lord send the twelve apostles to the lost sheep of the house of Israel, if these men did not know the truth? How also did the seventy preach, unless they had themselves previously known the truth of what was preached? Or how could Peter have been in ignorance, to whom the Lord gave testimony, that flesh and blood had not revealed to him, but the Father, who is in heaven? Just, then, as “Paul [was] an apostle, not of men, neither by man, but by Jesus Christ, and God the Father,” [so with the rest;] the Son indeed leading them to the Father, but the Father revealing to them the Son.
3. But that Paul acceded to [the request of] those who summoned him to the apostles, on account of the question [which had been raised], and went up to them, with Barnabas, to Jerusalem, not without reason, but that the liberty of the Gentiles might be confirmed by them, he does himself say, in the Epistle to the Galatians: “Then, fourteen years after, I went up again to Jerusalem with Barnabas, taking also Titus. But I went up by revelation, and communicated to them that Gospel which I preached among the Gentiles.” And again he says, “For an hour we did give place to subjection, that the truth of the gospel might continue with you.” If, then, any one shall, from the Acts of the Apostles, carefully scrutinize the time concerning which it is written that he went up to Jerusalem on account of the forementioned question, he will find those years mentioned by Paul coinciding with it. Thus the statement of Paul harmonizes with, and is, as it were, identical with, the testimony of Luke regarding the apostles.
Source: Adversus haereses (New Advent)