There is a logical fallacy known as the “No true Scotsman” fallacy. Here is how it is described in Wikipedia.
Philosophy professor Bradley Dowden explains the fallacy as an “ad hoc rescue” of a refuted generalization attempt. The following is a simplified rendition of the fallacy:
Person A: “No Scotsman puts sugar on his porridge.”
Person B: “But my uncle Angus likes sugar with his porridge.”
Person A: “Ah yes, but no true Scotsman puts sugar on his porridge.”
This fallacy is often used in debates about evolution and creation. The evolutionist claims that scientists believe in evolution. The creationist points out that a lot of scientists believe in creation and reject evolution. The evolutionist says that they are not true scientists.
Even Christians sometimes use this fallacy to dismiss other Christians with whom they disagree on doctrine. I was once engaged in an internet discussion with someone who believed that the only true Christians were those who believed in the five points of Calvinism. Anyone who rejected this belief was not a true Christian. (If you don’t know what Calvinism is, you can find out here:
https://carm.org/what-is-calvinism )
However this argument isn’t always a fallacy. It is possible that there are people who claim to be Scotsmen when they are not. There are non scientists who claim to be scientists. And there are definitely many who claim to be Christians when they are not. It is possible that false Christians outnumber true Christians. Jesus told a parable that explains why this is the case.
He put another parable before them, saying, “The kingdom of heaven may be compared to a man who sowed good seed in his field, but while his men were sleeping, his enemy came and sowed weeds among the wheat and went away. So when the plants came up and bore grain, then the weeds appeared also. And the servants of the master of the house came and said to him, ‘Master, did you not sow good seed in your field? How then does it have weeds?’
He said to them, ‘An enemy has done this.’
So the servants said to him, ‘Then do you want us to go and gather them?’
But he said, ‘No, lest in gathering the weeds you root up the wheat along with them. Let both grow together until the harvest, and at harvest time I will tell the reapers, “Gather the weeds first and bind them in bundles to be burned, but gather the wheat into my barn.”’”
Matthew 13:24-30 ESV
He explained the parable to his disciples.
Then he left the crowds and went into the house. And his disciples came to him, saying, “Explain to us the parable of the weeds of the field.”
He answered, “The one who sows the good seed is the Son of Man. The field is the world, and the good seed is the sons of the kingdom. The weeds are the sons of the evil one, and the enemy who sowed them is the devil. The harvest is the end of the age, and the reapers are angels. Just as the weeds are gathered and burned with fire, so will it be at the end of the age. The Son of Man will send his angels, and they will gather out of his kingdom all causes of sin and all law-breakers, and throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. He who has ears, let him hear.
Matthew 13:36-43 ESV
The weeds in this parable are darnel, a plant that resembles wheat when it first sprouts. Just as darnel is often mistaken for wheat, Satan’s false Christians are often mistaken for real Christians. Satan uses these weeds to lead true Christians into erroneous beliefs and practices. Jesus warned about this.
Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will recognize them by their fruits.
Matthew 7:15-20 ESV
The weeds that Satan sows don’t realize they are weeds but think they are really Christians. There was a time when I was one of them. As far as I recall I have always believed the Bible was true and that Jesus died and rose from the dead, but I didn’t understand that his death paid for my sins and eternal life could be mine if I trusted him. I believed that I must earn my salvation by doing good works. I attended a church that was liberal in theology and I don’t recall ever hearing a clear explanation of the gospel there. I got baptized and joined the church so I could begin the process of working my way into Heaven.
Shortly after this I heard and believed the gospel and became a genuine Christian but I sometimes wonder what my life would be like if this hadn’t happened. It is possible that I would have a blog today but instead of using it to teach the truth I would be spreading false teaching and turning others from the faith. Paul warned that we should make sure of our own salvation.
Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test!
2 Corinthians 13:5
Perhaps a good way to do this self-examination would be to study 1 John. It was written to assure believers that they have eternal life.
I write these things to you who believe in the name of the Son of God, that you may know that you have eternal life.
1 John 5:13 ESV
If you find that some of the things it says aren’t true of you that might be an indication that you aren’t yet a Christian and that you need to be saved.