You will know them by their fruits. Grapes are not gathered from thorn bushes, nor figs from thistles, are they? Even so, every good tree bears good fruit; but the bad tree bears bad fruit. A good tree cannot produce bad fruit, nor can a bad tree produce good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. So then, you will know them by their fruits. (Matthew 7:16–20)
After warning about false prophets, Jesus tells us what to watch for in identifying them. Because they are so extremely deceptive and dangerous ravenous spiritual and moral wolves in sheep’s clothing-the Lord would hardly have left us without means of determining who they are.
Jesus assures us that we will know them by their fruits. A fruit tree may be beautiful, decorative, and offer pleasant shade in the summer. But its primary purpose is to bear fruit, and it is therefore judged by what it produces and not by how it looks. (That understanding is the key to interpreting John 15 properly.)