John Calvin Commentary 1 John 4:14

"And we have beheld and bear witness that the Father hath sent the Son [to be] the Saviour of the world." — 1 John 4:14 (ASV)

And 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 therefore follows that he is received by us through faith. For the Apostle's design is to show that God is so united to us by faith and love that he truly dwells in us and renders himself, in a way, 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 rather the seeing that belongs to faith, by which they recognized the glory of God in Christ, in accordance with the words that follow: that he was sent to be the Savior of the world; and this knowledge flows from the illumination of the Spirit.