Hey–––Yes, people can be self-deceived. But this is not some innocent or unintentional white lie to yourself though, it is a deliberate, carelessness that ignores truth for self -gain or gratification. It is not self-sacrificing or submissive to God. This is testified throughout the NT. Jesus says, “Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers! Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock.” (Matthew 7:22-24) Notice that these are all verbal claims “in your [Christ’s] name”. There is a reason why the apostles do not just say to those who verbally accept Christ, ‘You’re saved! That’s it –– it’s done. Don’t worry about it’. In fact, Paul says to the Corinthians: “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 13:5-6) There is good reason why the NT is chock-full of warnings not to be self-deceived.

In short, people may say that they have Christ in their heart, but in actuality, they have only “said so”. They have not allowed the Holy Spirit to penetrate and transform their heart, soul, and mind, which in turn changes behaviour and attitude. Their actions do not follow what they claim to believe. What they actually believe, then, is that they are ‘good enough’ to get into heaven, which strives against the gospel: “for all have sinned and fall short of the glory of God.” (Romans 3:23)

But this is not a black-and-white issue, for humans at least. You may some other concerns or questions related to this that may pop up in your head, and for that I recommend reading this Q&A: Q10. Can you lose your salvation or are you eternally secure?

Hope that helps–––God bless!