Albert Barnes Commentary Matthew 12:33

"Either make the tree good, and its fruit good; or make the tree corrupt, and its fruit corrupt: for the tree is known by its fruit." — Matthew 12:33 (ASV)

Either make, etc. The fact asserted in this verse is, that a tree is known not by its leaves, or bark, or form, but by its fruit. The application to the argument is this: "You are to judge of man's being in league with Satan by his works. If my doctrines and works are properly the works of Satan, then I am corrupt; if not, then your charge is blasphemy. So, on the other hand, if, notwithstanding your professions, your works are the works of the devil, and your doctrines are such as he would teach, it would prove concerning you what you charge against me." In this indirect but powerful manner, he advances to the charge against them, which he urges in the following verses.