1 John 1:3

Wherefore he also exhorts us to believe in the name of His (the Father's, ) Son Jesus Christ, that "our fellowship may be with the Father, and with His Son Jesus Christ." Paul, in like manner, everywhere speaks of "God the Father, and our Lord Jesus Christ.

Those saw the Lord Himself present in the flesh, and heard words from the mouth of the Lord, and told them to us. Consequently we also have heard, but have not seen. Are we then less happy than those who saw and heard? And how does he add, "That ye also may have fellowship with us"? Those saw, we have not seen, and yet we are fellows; because we hold the faith in common. For there was one who did not believe even upon seeing, and would needs handle, and so believe, and said, "I will not believe except I thrust my fingers into the place of the nails, and touch His scars." And He did give Himself for a time to be handled by the hands of men, who always giveth Himself to be seen by the sight of the angels; and that disciple did handle, and exclaimed, "My Lord, and my God!" Because he touched the Man, he confessed the God. And the Lord, to console us who, now that He sitteth in heaven, cannot touch Him with the hand, but only reach Him with faith, said to him, "Because thou hast seen, thou hast believed; blessed are they that have not seen, and yet believe." We are here described, we designated. Then let the blessedness take place in us, of which the Lord predicted that it should take place; let us firmly hold that which we see not; because those tell us who have seen.

Our fellowship is in the unity of our faith here on earth and in the eternal dwelling place of God in heaven.

"What we have seen and heard, we also declare to you, so that you may have fellowship with us, and our fellowship is with the Father and His Son Jesus Christ."
And what profit do we bring you from this announcement? Namely, that just as we have taken you as partners [κοινωνοὺς] through the words of what we have seen and heard, so you may also be partners of the Father and of his Son Jesus Christ.

What did they proclaim, but that eternal life has appeared to us and that we have become witnesses of it? What you gain from this proclamation is the right to share this experience with us. For the one who is in fellowship with us has fellowship with the Father and with his Son, Jesus Christ, and since you share in the same fellowship, we shall all have joy together, in that we have been united to God.

What we have seen and heard, we proclaim to you, etc. Blessed John clearly shows that anyone who desires to have fellowship with God must first be united with the fellowship of the Church, learn that faith, and be imbued with its sacraments, which the disciples received from the Truth dwelling in the flesh itself. Nor in any way are those less associated with God who believe through the doctrine of the apostles, than those who believed through the Lord preaching in the world himself, except insofar as the quality of faith or works distinguishes them. Hence also concerning this fellowship of the saints, which they have in the Father and the Son, the Son himself, praying to the Father, says: Holy Father, keep them in your name, which you have given to me, that they may be one as we are one (John XVII). And a little later: I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us (John XVII).

What exactly? That the Life, being eternal, appeared to us, and we were eyewitnesses of it both before the cross and after the resurrection. For one and the same was nailed in the flesh to the cross and rose again in the same flesh. And what benefit is it to you, he says, that we proclaim this to you? This: that just as through the word we receive you as partakers of what we have seen and heard, so we have you as partakers of both the Father and His Son Jesus Christ, and having received this, we who have cleaved to God can be filled with joy.