What we have seen: The Word of life had, as a human being, revealed God to the apostles.
this fellowship of ours is with the Father and with His Son: After the Spirit gave the apostles new life (regenerated them), they entered spiritual fellowship with the Father and the Son.
you also may have fellowship with us: Whoever entered into fellowship with the apostles by associating with them while they were alive, or enters now by remaining in their teaching, has fellowship with the Father and the Son through them.