One

Irenaeus of Lyon

"There is one body and one Spirit, and God chose you to have one hope. There is one Lord, one faith, and one baptism. There is one God and Father of us all, who rules over everyone. He works through all of us and in all of us."

(Letter to the Ephesians chapter 4 verses 4 - 6, ERV)

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Irenaeus was a leader, thinker, and writer in the early Christian Movement. He had grown up in the church at Smyrna (in today's Turkey) led by Polycarp, a legend in the early church. Polycarp had been a student of John the Apostle himself, and he passed on stories and teachings from John and other "elders" in the Movement. Irenaeus absorbed it all with the fabulous mind he had. As an adult he moved east to Lyons, France where he cared for the Movement's outpost there and handed on the 'deposit of faith' as he had heard it from Polycarp, who had received it from St. John the Apostle who received it from... Well, I imagine you can see what makes Irenaeus so important in the history of the Christian Movement.

Today, Irenaeus describes what was in the revelation Jesus entrusted his Apostles with, and how carefully it was handed on.

Now the Church, although scattered over the whole civilized world to the end of the earth, received from the apostles and their disciples its faith in one God, the Father Almighty, who made the heaven, and the earth, and the seas, and all that is in them, and in one Christ Jesus, the Son of God, who was made flesh for our salvation, and in the Holy Spirit, who through the prophets proclaimed the dispensations of God—the comings, the birth of a virgin, the suffering, the resurrection from the dead, and the bodily reception into the heavens of the beloved, Christ Jesus our Lord, and his coming from the heavens in the glory of the Father to restore all things, and to raise up all flesh, that is, the whole human race, so that every knee may bow, of things in heaven and on earth and under the earth, to Christ Jesus our Lord and God and Saviour and King, according to the pleasure of the invisible Father, and every tongue may confess him, and that he may execute righteous judgment on all. 

The spiritual powers of wickedness, and the angels who transgressed and fell into apostasy, and the godless and wicked and lawless and blasphemers among men he will send into the eternal fire. But to the righteous and holy, and those who have kept his commandments and have remained in his love, some from the beginning [of life] and some since their repentance, he will by his grace give life incorrupt, and will clothe them with eternal glory.

Having received this preaching and this faith, as I have said, the Church, although scattered in the whole world, carefully preserves it, as if living in one house. She believes these things [everywhere] alike, as if she had but one heart and one soul, and preaches them harmoniously, teaches them, and hands them down, as if she had but one mouth. For the languages of the world are different, but the meaning of the [Christian] tradition is one and the same. Neither do the churches that have been established in Germany believe otherwise, or hand down any other tradition, nor those among the Iberians, nor those among the Celts, nor in Egypt, nor in Libya, nor those established in the middle parts of the world.

Irenaeus of Lyons (early 2nd century – c. AD 202)
Against All Heresies book 1 chapters 10 sections 1 - 2 (written about AD 180)