Here one has to read John 3:13 in context. Jesus is saying "No human being can speak from personal knowledge, as I do, who came from heaven."
So we, speaking of the secrets of the future world, should very naturally say: "No man has been there to tell us about them." In saying this, we do not deny that any one has actually entered the eternal world, but merely that any one has gone thither, and returned to unfold its mystery."