Matthew Poole and continuators on 1 John 1:3

3 That which we have seen and heard, declare we unto you, that ye also may have fellowship with us , and truly our fellowship is with the Father, and with his son Jesus Christ .

He now proceeds with what he intended, not only professing to testify most certainly known things (which he further with great earnestness inculcates) but declaring also the end of this Testimony; viz. Not meerly, that they to whom he writes might know them too (as if the being a Christian, did only stand in having some peculiar notions from other Men, and that they were only to know for knowings sake) but that they might have fellowship, i. e. partake and communicate with them (viz. the Apostles, and the whole community of living Christians) in all the Vital Influences, Holy Practice, the Dignities, Pleasures, and Consolations belonging to the Christian State; whereupon he adds, and truly, &c.

q. d. Nor are the advantages of that State, in their kind and nature, Terrene, Sensual, Secular, but Divine and Heavenly, such as are imparted to us by the Father and his Son Iesus Christ; or, wherein we are truly said to participate, and have a Communion with them. That Blessed Spirit, who is the immediate Author to us of all gracious communication (whence this is also styled the Communion of the Holy Ghost, 2 Cor. 13, 14.) being in reality, the Spirit of the Father and the Son.