Most assuredly I tell you, we speak that which we know, and testify of that which we have seen, and you don't receive our witness.
That we have seen - Jesus had seen by his omniscient eye all the operations of the Spirit on the hearts of men. His ministers have seen its effects as we see the effects of the wind, and, having seen men changed from sin to holiness, they are qualified to bear witness to the truth and reality of the change. Every successful minister of the gospel thus becomes a witness of the saving power of the gospel.