Jesus speaks of heavenly things from firsthand heavenly knowledge.

John 3:11–13

¹¹Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.

¹²If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?

¹³And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.