The Lord of all bestowed on his apostles the power of proclaiming the Gospel. And it is through them that we have come to understand the truth, that is to say the teaching of the Son of God. To them the Lord said: “Whoever listens to you, listens to me. Whoever rejects you, rejects me and rejects the one who sent me” (Lk 10,16). For we have not known the message of salvation from any besides those who brought the Gospel to us.
This is the Gospel they first of all preached. Then, by God’s will, they handed it on to us in their Writings so that it might become “the pillar and foundation” of our faith (1Tm 3,15). It is not permissible to say they preached before they had acquired perfect understanding, as some people rashly claim who boast of being correctors of the apostles. For indeed, after our Lord had been raised from the dead and the apostles had been “clothed with power from on high” (Lk 24,49) by the coming of the Holy Spirit, they were filled with assurance about everything and possessed perfect knowledge. Then they went forth “to the ends of the world” (Ps 19,5; Rom 10,18), proclaiming the Good News of all the good things that have come to us from God and announcing peace on earth to men. Each and all possessed the Gospel of God in equal measure.
by Saint Irenaeus of Lyons (c.130-c.208), Against the heresies, III, 1