From the treatise On the Prescription of Heretics by Tertullian, priest
The preaching of the Apostles
Our Lord Jesus Christ himself declared what he was, what he had been, how he was carrying out his Father’s will, what obligations he demanded of men. This he did during his earthly life, either publicly to the crowds or privately to his disciples. Twelve of these he picked out to be his special companions, appointed to teach the nations.
The only way in which we can prove what the apostles taught – that is to say, what Christ revealed to them – is through those same churches. They were founded by the apostles themselves, who first preached to them by what is called the living voice and later by means of letters.
The Lord had said clearly in former times: I have many more things to tell you, but you cannot endure them now. But he went on to say: When the Spirit of truth comes, he will lead you into the whole truth. Thus Christ shows us that the apostles had full knowledge of the truth, for he had promised that they would receive the whole truth through the Spirit of truth. His promise was certainly fulfilled, since the Acts of the Apostles prove that the Holy Spirit came down on them.