1 John 4:14

14And we have seen He now explains the other part of the knowledge of God, which we have referred to, that he communicates himself to us in his Son, and offers himself to be enjoyed in him. It hence follows, that he is by faith received by us. For the design of the Apostle is to shew, that God is so united to us by faith and love, that he really dwells in us and renders himself in a manner visible by the effect of his power, who otherwise could not be seen by us.

When the Apostle says, We have, seen and do testify, he refers to himself and others. And by seeing, he does not mean any sort of seeing, but what belongs to faith by which they recognized the glory of God in Christ, according to what follows, that he was sent to be the Savior of the world; and this knowledge flows from the illumination of the Spirit.