Adam Clarke on John 3:12

If I have told you earthly things - If, after I have illustrated this new birth by a most expressive metaphor taken from earthly things, and after all you believe not; how can you believe, should I tell you of heavenly things, in such language as angels use, where earthly images and illustrations can have no place? Or, if you, a teacher in Israel, do not understand the nature of such an earthly thing, or custom of the kingdom established over the Jewish nation, as being born of baptism, practised every day in the initiation of proselytes, how will you understand such heavenly things as the initiation of my disciples by the baptism of the Holy Ghost and fire from heaven, if I should proceed farther on the subject?