Albert Barnes on 1 John 4:14

And we have seen. 1Jn 1:1.

And do testify. 1Jn 1:3. That is, we who are apostles bear witness to you of this great truth, that God has sent his Son to be a Saviour. Comp. Jn 20:31. The reason why this is referred to here is not quite apparent, but the train of thought in this passage would seem to be this: The writer is discoursing of the love of God, and of its manifestation in the gift of the Saviour, and of the proper influence which it should have on us. Struck with the greatness and importance of the subject, his mind adverts to the evidence on which what he was saying rested--the evidence that the Father had really thus manifested his love. That evidence he repeats, that he had actually seen him who had been sent, and had the clearest demonstration that what he deemed so important had really occurred.