Not believing in Jesus. I could simply stop there, but I’m a pastor, a preacher so you know I won’t. The question you should ask is, why?
Jesus died for the sins of the whole world (1 John 2:2). Jesus died for all our sins past, present and future (Hebrews 10:12-14). Jesus blood atonement was perfect and gives us the light of life by cleansing us from all our sins (John 8:12; 1 John 1:7). If these are true statements, then one might presume there is no unpardonable sin, that there is no sin Jesus Christ did not die for. That my friends is the challenge.
How often do we wonder how someone can be so evil, do such a horrible thing, be so wicked or hateful? We do so from a Christian viewpoint. We, in our walk with Christ, want to esteem others higher than ourselves. Sometimes when we do, we forget that regardless, those without Christ are the children of Satan. They will do the will of their father who is Satan (John 8:44). They cannot help but be evil.
That is the key my dear friends. If one does not accept Jesus as their savior, they have not received the atonement even if it is already out there waiting for them. Like the present under the tree, it is there but one must take it to possess it. Though it is freely given, the work all done by the Savior himself, the grace of God extends it to His chosen children, the gift must be received. God can and does open hearts for that reception, but God does not want robots in His kingdom.
What is the unpardonable sin? To deny Christ (John 16:9 ). This makes His sacrifice vain, it denies the Father’s selfless mercy that gave His son, and it makes the Holy Spirit a liar (1 John 5:10). Accept Jesus the perfectly complete atonement for all the sins of mankind.