Even staunch Calvinist believe that assurance can be false.
From R.C. Sproul
Probably the most prevalent doctrine of justification in modern culture is the doctrine of justification by works. Most Americans, including those who call themselves evangelicals, believe that people will get into heaven if they live a good life. They think: “I am pretty good.I do the best that I can, and haven’t done anything terribly wrong. God will be satisfied.” Christians may think: “I look forward to the day when I will be welcomed into heaven. After all, I’ve gone to church for forty years, I have been a Sunday school teacher, I’ve given money to the church and other good causes, and I’ve never murdered anyone or committed adultery.” The confidence of such people is based upon their own goodness, which is not the biblical understanding of salvation.
This false concept of salvation is pervasive in our culture, and it is as pernicious as it is pervasive, because it completely obscures and denies the true gospel. What is so damaging about it is that it gives people a false sense of security. Jesus warns us about that in the Sermon on the Mount (Matt.5–7). Near the end of the sermon, He says of the day of judgment, “On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’” (
Matt.7:22). They do not say “Lord” twice for emphasis but for intimacy. About fifteen times in the Bible, the personal form of address is used twice to express a deep level of personal intimacy. That is why this warning of Jesus is so terrifying. Jesus is saying that people are going to come to Him on the last day not only confessing Him as Lord but also claiming an intimate, personal relationship with Him. He is talking about people who will approach the judgment with a strong sense of assurance of their state of grace and of their personal relationship with Jesus Christ. They will claim to have cast out demons in His name, to have preached, to have worked, to have been engaged in the missionary activity of the church, and to have been involved in evangelism. But Jesus says, “And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness’” (v.23).
Can a person delude himself about the state of his faith? Yes. Can we think we have saving faith and not have it? Can we have it and not be sure we have it? Yes, we can really have it and not be sure that we have it. We can really not have it and be sure we do have it. That is why we have to study the Scriptures diligently and prayerfully, so that we can learn to distinguish between true assurance and false assurance.