Jamieson, Fausset and Brown on 1 John 4:14

14. And we--primarily, we apostles, Christ's appointed eye-witnesses to testify to the facts concerning Him. The internal evidence of the indwelling Spirit (1Jo 4:13) is corroborated by the external evidence of the eye-witnesses to the fact of the Father having "sent His Son to be the Saviour of the world."

seen--Greek, "contemplated," "attentively beheld" (see on 1Jo 1:1).

sent--Greek, "hath sent": not an entirely past fact (aorist), but one of which the effects continue (perfect tense).