The Bible says that on the judgment day it will be revealed that some who think they are Christians are really not. In the Sermon on the Mount, in Matthew 7:21-23, Jesus said,

“Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven; but he who does the will of My Father who is in heaven. 22 “Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ 23 “And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.’

This may seem to you to be a very strange thing, that some people will be so self-deceived, that they think they are Christians when they are not. How can this happen? How does someone not know that he is not a Christian? How does he or she live so much of a life thinking that they are Christians when they are not?

The only answer is that they haven’t really known what a Christian is. They evidently have allowed false prophets to influence them; and they, at the beginning, entered not at the narrow gate but at the broad gate and went down and lived in the broad way—the way of the world. And all along in their journey, they never at any point got to know the Lord and to be changed by Him. What a tragic deception.

But how is it that they are so deceived?  And also, how is it that true Christians are also deceived into thinking that some false Christians are true Christians? Here are a few reasons why:

• Knowing Him does not mean saying the right words. It is possible for a person to say all the right words, to have a very holy and righteous speech, and yet not be a Christian. A changed language does not mean that there is a changed heart.
• Some unbelievers may use Scripture and Scriptural teaching as a kind of philosophy, but they don’t really know Christ or are converted.
• Some may speak and even preach the Scriptures fervently, but it may be entirely of the flesh and not for the sake of Christ.

The test of course is whether a person has the fruit of the Spirt, and also whether they know Him or not. Do they pray and read their bible regularly? Do they have a growing love relationship with Him?