John Calvin Commentary 1 John 1:3

"that which we have seen and heard declare we unto you also, that ye also may have fellowship with us: yea, and our fellowship is with the Father, and with his Son Jesus Christ:" — 1 John 1:3 (ASV)

That which we have seen. He now repeats for the third time the words seen and heard, so that nothing might be lacking regarding the real certainty of his doctrine. And it should be carefully noted that the heralds of the Gospel chosen by Christ were those who were suitable and faithful witnesses of all those things they were to declare. He also testifies to their heartfelt motivation, for he says that he was moved to write by no other reason than to invite those to whom he was writing to share in an inestimable good. From this it appears how much care he had for their salvation, which significantly helped to induce them to believe; for we would be extremely ungrateful if we refuse to listen to him who wishes to share with us a part of that happiness which he has obtained.

He also presents the fruit received from the Gospel: namely, that through it we are united to God, and to His Son Christ, in whom the chief good is found. It was necessary for him to add this second point, not only to represent the doctrine of the Gospel as precious and lovely, but also to show that he wished them to be his associates for no other purpose than to lead them to God, so that they might all be one in Him. For the ungodly also have a mutual union among themselves, but it is without God; in fact, it leads them to alienate themselves more and more from God, which is the worst of all evils. Indeed, as has been stated, our only true happiness is to be received into God’s favor, so that we may be really united to Him in Christ, of which John speaks in his Gospel (John 17).

In short, John declares that just as the apostles were adopted by Christ as brothers, so that, being gathered into one body, they might together be united to God, so He does the same with other colleagues; though many, they are nevertheless made partakers of this holy and blessed union.