1 John 1:3

That which we have seen and heard--resumed from Jo1 1:1, wherein the sentence, being interrupted by Jo1 1:2, parenthesis, was left incomplete. declare we unto you--Oldest manuscripts add also; unto you also who have not seen or heard Him. that ye also may have fellowship with us--that ye also who have not seen, may have the fellowship with us which we who have seen enjoy; what that fellowship consists in he proceeds to state, "Our fellowship is with the Father and with His Son." Faith realizes what we have not seen as spiritually visible; not till by faith we too have seen, do we know all the excellency of the true Solomon. He Himself is ours; He in us and we in Him. We are "partakers of the divine nature." We know God only by having fellowship with Him; He may thus be known, but not comprehended. The repetition of "with" before the "Son," distinguishes the persons, while the fellowship or communion with both Father and Son, implies their unity. It is not added "and with the Holy Ghost"; for it is by the Holy Ghost or Spirit of the Father and Son in us, that we are enabled to have fellowship with the Father and Son (compare Jo1 3:24). Believers enjoy the fellowship OF, but not WITH, the Holy Ghost. "Through Christ God closes up the chasm that separated Him from the human race, and imparts Himself to them in the communion of the divine life" [NEANDER].

— Jamieson-Fausset-Brown Commentary (public domain)