The disciples of Christ are known each other, and they are known each other by their fruits, in the same way, those who are not His disciples are also known in the same way, and Jesus warns us about this so we do not let ourselves be deceived. The Bible tells us what the true gospel of Christ is and we must study the Word so as not to be deceived by false prophets.
Jesus told us:
15 Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves.
16 By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles?
17 Likewise, every good tree bears good fruit, but a bad tree bears bad fruit.
Christ Himself commands us to guard against false prophets, those who come to us with something completely different from the gospel, offering us things that Christ has never offered us, and the only way for us to know when they are bringing us something that is not Gospel is reading and studying the Holy Scriptures.