Jesus just finished telling us there are two paths you can take, the narrow path to the narrow gate of life or the broad path to the wide gate of destruction. "Few are those who find" that narrow gate. Why? Because there are so many false prophets in our midst who deceive people into taking the broad path.
"They come to you in sheep's clothing but inwardly are ravenous wolves." In other words they take on the appearance of being one of Christ's own, but their true desire is to prey on you. They claim to be Christians but they are motivated by a fleshly appetite. They are good at gaining people's trust and using it for their own ends.
Notice Jesus doesn't say we should try to discern false prophets by determining if they are sincere or not. 2 Timothy 3:13 says these imposters are always "deceiving and being deceived." They have already convinced themselves of their own deception and will therefore come across as very sincere. Neither does Jesus encourage us to engage them in theological debate. Paul warned Timothy "not to wrangle about words" (2 Timothy 2:14) when dealing with the false teachers who denied the resurrection. Such arguments only confuse and upset those who are listening in.
Rather Jesus says, "You will know them by their fruits." You are not responsible for knowing someone's heart or mind, because that's impossible. Instead all you need to do is observe their outward conduct, which will reveal everything you need to know about their inner person. Just as a good tree bears good fruit and a bad tree bad fruit, a good person will behave uprightly and a bad person will behave badly. It should be obvious enough for even a child to figure out.
What sort of bad stuff are we talking about? If you read through chapter 2 of 2 Peter you'll get an overview of what Jesus might be referring to. False prophets are described as sensual, greedy, indulgent, reckless and arrogant. They are adulterers, exploiters, boasters, liars and deceivers. What's worse is that they have been graced to some extent with the knowledge of Jesus Christ, just enough for temporary reform and an ability to speak with seeming credibility about spiritual things. But before long they will return to their old ways and attempt to ensnare others along with them (2:20-22).
I think one of the main reasons why it's difficult to apply Jesus' "bad tree bears bad fruit" principle to real life situations is that false prophets/teachers are great talkers, and we tend to believe what people say about themselves rather than think to check their words against their actions. And when a leader's words and actions contradict one another, we automatically think we are being "uncharitable" rather than see conduct as the true indicator of their inner character.
If you don't believe you fall into this trap, consider the leaders in the church you think most highly of. Do you hold that opinion of them because of how they speak, or because you have carefully observed how they actually conduct themselves? Most of us would have to admit that we are immediately taken in by the impressive speaker who preaches well and talks piously. Rarely do we think, "Okay, well, that's great talk and I really love listening to him, but how does he treat people?" Likewise, it's rare for us to notice that person who is above reproach in all his conduct but hardly says a word.
I'm not saying that church leaders shouldn't have marriage problems and aren't allowed to be as weak and human as the rest of us. I'm only suggesting that when it comes to choosing our leaders, we tend to be biased toward people who speak impressively. We are naturally prone to doing the opposite of Jesus' advice, which is that we ought to be following the person who bears fruit, not just talks great talk. And he gives this warning because there are many false prophets out there who are looking to exploit this very weakness of ours. There's a reason why so few find the narrow gate.