15 “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. 16 You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? 17 So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. 18 A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Thus you will recognize them by their fruits.
At work, I often teardown and reassemble laptops, tablets, and all in one PC’s for repairs using detailed instructions, whether on paper or instructional videos. One of the things that we ALWAYS do before we remove one screw or panel is to verify that we have the instructions for the right model. Because different models have different builds, and if the instructions don’t match, you risk destroying a client’s device.
This is the point of verses 15-20. Someone may be teaching something that sounds good, but is just a little bit off what the Bible really says. How is the ONLY way that you can know? Be so familiar with the Word that you can spot an incorrect teaching from a mile away.
Could you on the spot state or recognize differences between Christianity and Mormonism, or Jehovah’s Witness Doctrines? What about Word of Faith teachings? You see, it’s easy to see the major differences between Christianity and the Muslim or Jewish faiths, but the closer it gets to “Orthodox Christianity”, the more difficult it becomes.
The only way you can know is to line it up with what the Bible actually says. The more you read and study the Bible, the more you know it, the better you can discern what is true and what is not.