John 3:12

If I have told you earthly things
Not that the doctrines he delivered were earthly ones; for he was not of the earth, but from heaven, and above all, and so spake not of the earth, but of heaven, ( John 3:31 ) ; and this doctrine of regeneration was an heavenly doctrine; and the thing itself required supernatural power, and grace from above: but either they were the more easy doctrines of the Gospel; or were delivered in a plain and easy style, and illustrated by similes taken from earthly things, as from human birth, from the water, and from the wind:

and ye believe not;
i.e. those things; ye do not receive them, nor give credit to them; or "me", as the Ethiopic Version adds, who relate them on the best evidence, having fully known, and clearly seen them:

how shall ye believe;
give credit to me, or receive my testimony:

if I tell you of heavenly things?
of the more sublime doctrines of the Gospel, such as the descent of the Messiah from heaven; the union of the two natures, human and divine, in him; his being the only begotten Son of God; his crucifixion and death, signified by the lifting up of the serpent on a pole in the wilderness; and the wonderful love of God to the Gentile world in giving Christ to, and for them; and the salvation, and eternal happiness of all that believe in him, whether they be Jews or Gentiles; and these delivered in language suitable to them, without figures, or natural similes, which help the understanding, and convey ideas of things more easily to it.