NKJ Matthew 7:16 "You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles?
A friend recently asked me about this passage (particularly about the meaning of "fruit") and part of my reply included this material by Carson. I may as well put it on my blog as well:
"One's "fruit"—not just what one does, but all one says and does—will ultimately reveal what one is (cf. James 3:12). The Semitic way of expression (i.e., both positive and negative—viz., every good tree bears good fruit, no good tree bears bad fruit, etc.) makes the test certain, but not necessarily easy or quick. Living according to kingdom norms can be feigned for a time; but what one is will eventually reveal itself in what one does. However guarded one's words, they will finally betray him (cf. 12:33-37; Luke 6:45). Ultimately false prophets tear down faith (2 Tim 2:18) and promote divisiveness, bitterness (e.g., 1 Tim 6:4-5; 2 Tim 2:23), and various kinds of ungodliness (2 Tim 2:16). Meek discernment and understanding the dire consequences of the false prophets' teaching are needed. But at the same time censoriousness over minutiae must be avoided."
D. A. Carson, "Matthew," in The Expositor's Bible Commentary, ed. Frank E. Gæbelein (Zondervan, 1984), 8:191.