If I told you earthly things and you don't believe, how will you believe if I tell you heavenly things?
Heavenly things ... is a reference to such things as the incarnation, the death of Christ for the sins of the world, the existence of the spirit world above our own, the final judgment, heaven, hell, and all of those great spiritual realities lying utterly beyond earthly vision.  It was of some of such heavenly things that Jesus proceeded to speak to Nicodemus.