That which we have seen and heard, report we to you, in order that you too might have fellowship with us and moreover, our fellowship with the Father and with His Son Jesus Christ.1 John 1:3
This verse introduces one of the major themes of Johnâ€™s epistle. That theme is that the manifested Word which John was a witness of (i.e. had seen and heard, vs. 1-2) is being reported to them in order that they might have fellowship with both the witnesses of the Word made Flesh, and with the Father and His Christ.
Note how John has gradually linked the Word (logos) with eternal life (zoe) and now makes explicit what has been implicit. The Word of Life made manifest is none other than Jesus Christ. But another claim is made here, that of Christâ€™s sonship. We also see how for John, theology and philosophy are servants to fellowship, not vice versa.
Johnâ€™s readers were confronted with a perceived problem regarding Jesusâ€™ relationship to God. John gets right to the point: Jesus was the Christ and was the Son of the Father, and was the Word, and was the Life, and was there from the beginning. And none of that is contradictory. Very simply, he leaves no wiggle room to heretical notions of Christâ€™s humanity or divinity.
But notice too the vital tone of familial relation (i.e. Father to Son). John will revisit multiple times in this epistle the importance of fellowship with God (through Christ), and the resulting fellowship we enjoy with our brother. The letterâ€™s purpose statement in this verse (and the next) is presented as an invitation to the readers to enjoy union (fellowship) with the Father and His Son. This is both a safeguard against the heresies which the early church was tempted by, and a boon of encouragement in the persecution and distresses they faced.