Chapter I.—The apostles did not commence to preach the Gospel, or to place anything on record until they were endowed with the gifts and power of the Holy Spirit. They preached one God alone, Maker of heaven and earth.
1. We have learned from none others the plan of our salvation, than from those through whom the Gospel has come down to us, which they did at one time proclaim in public, and, at a later period, by the will of God, handed down to us in the Scriptures, to be the ground and pillar of our faith. See 1 Timothy 3:15, where these terms are used in reference to the Church.For it is unlawful to assert that they preached before they possessed “perfect knowledge,” as some do even venture to say, boasting themselves as improvers of the apostles. For, after our Lord rose from the dead, [the apostles] were invested with power from on high when the Holy Spirit came down [upon them], were filled from all [His gifts], and had perfect knowledge: they departed to the ends of the earth, preaching the glad tidings of the good things [sent] from God to us, and proclaiming the peace of heaven to men, who indeed do all equally and individually possess the Gospel of God. Matthew also issued a written Gospel among the Hebrews in their own dialect, while Peter and Paul were preaching at Rome, and laying the foundations of the Church. After their departure, Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter. Luke also, the companion of Paul, recorded in a book the Gospel preached by him. Afterwards, John, the disciple of the Lord, who also had leaned upon His breast, did himself publish a Gospel during his residence at Ephesus in Asia.
2. These have all declared to us that there is one God, Creator of heaven and earth, announced by the law and the prophets; and one Christ the Son of God. If any one do not agree to these truths, he despises the companions of the Lord; nay more, he despises Christ Himself the Lord; yea, he despises the Father also, and stands self-condemned, resisting and opposing his own salvation, as is the case with all heretics.
~Irenaeus, from Against Heresies: Book III