Parallel passage: Luke 6:43 (cf. also infra, Matthew 12:33). A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. This correspondence of external product to internal character is necessary.
It cannot (emphatic) be otherwise. It' the heart is good, good results follow; therefore, he implies, if good results are not seen in these "false prophets," it is because of their real character. A bad life cannot but spring from a worthless heart.
Of course, our Lord deals only with the general rule. There are apparent anomalies in the world of spirit as of nature. Bring forth … bring forth; ἐνεγκεῖν (Westcott and Herr)… ποιεῖν. A good tree cannot have bad fruit hanging on it; a rotten or worthless tree cannot, with all its efforts, produce good fruit.