Charles Spurgeon Commentary 1 John 4:13-14

"hereby we know that we abide in him and he in us, because he hath given us of his Spirit. And we have beheld and bear witness that the Father hath sent the Son [to be] the Saviour of the world." — 1 John 4:13-14 (ASV)

Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit. And we have seen and do testify that the Father sent the Son to be the Saviour of the world.

John is speaking for himself and the rest of the apostles; for they were eyewitnesses who had seen Christ, and therefore could testify to him.