What does Matthew 7:16 mean?

Jesus has warned that false prophets may disguise themselves to look like sheep when they are in truth ravenous wolves (Matthew 7:15). This is yet another example of how the Bible calls believers to sensible, informed, mature faith (Acts 17:11; 1 John 4:1). You can't judge a self-proclaimed prophet by their outward appearance. Appearing smart, authoritative, moral, or "nice" does not mean what they say is true. Rather than judging shallowly, Jesus commands believers to use right judgment (John 7:24). When it comes to self-proclaimed teachers or prophets, that means looking at what comes from their teaching and from their lives.

Grapes and figs were common fruit in the diets of Jesus' listeners. People learned early on to recognize that the little berries on thorn bushes were not grapes on grapevines; the flowers on thistles were different from the petals on a fig tree. Parallel to this, people may appear impressive at first glance. They may come across as very religious and holy. Time, however, will reveal that person's character. Do their actions match their teaching? Do they give care to others when nobody is watching? Are those who follow their teachings people of good reputation? In the case of a supposed prophet, do their prophecies come true? Do they fit with what is known from Scripture?

If the plant does not produce grapes, it is not a grapevine. Or, it is diseased, according to Jesus in the following verse (Matthew 7:17).