https://www.mljtrust.org/sermons-online/acts-8-13/the-false-believer/

In a sermon on the authority of the believer, Dr. Martyn Lloyd-Jones asks: can a person perform the actions of a believer, yet not be saved? There are many people in the church who mistakenly believe they are saved because they were born into a certain family, uttered some phrases, or even were baptized. In spite all these things, they have never truly been redeemed. These people are deceived into thinking that they are true Christians merely by their external actions. This is one of the greatest threats to Christianity in all ages and periods of history. This is no less true in the early church as seen in the book of Acts. It is there that we meet Simon the magician. He heard the gospel, confessed faith, and was baptized. Later it is revealed that he was a false convert, exposed by his desire to buy power from the Holy Spirit. This is why Christians must test themselves and examine their fruit in order to see if their faith is true and not false like that of Simon. Faith can be tested by examining not only one’s confession of faith, but the fruit of one’s life. True believers will walk in the light, submit to Scripture, and love their brothers and sisters.