Matthew 12:31, Mark 3:29, and Luke 12:10 tell us that there is an unforgivable sin. Jesus says in Matthew “Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the [Holy] Spirit will not be forgiven men.” Whoa! So there is a sin that will not be forgiven! When Christians hear this for the first time, they generally will become anxious and want to know what the unforgivable sin is. Why not? We need to make sure that we will not commit it.
However, believers can calm down and breathe easy. To begin with, there are many passages that clearly tell us that Christians are forgiven of all their sins (Acts 13:39; Titus 2:14; 1 John 1:7, 9). So, if we are forgiven of our sins, then how can there be an unforgivable sin? The answer is simple when one stops and thinks about it.
When someone accepts Jesus Christ as their Lord and Savior, they receive the gift of the Holy Spirit. If you reject Jesus, then you do not receive the Spirit. Essentially, you are rejecting the Holy Spirit. To commit blasphemy against the Holy Spirit is to reject Jesus Christ. If one dies without accepting Jesus, then one dies without receiving the Holy Spirit. Thus, that person will die without God and will not receive salvation.
Scholar G.M. Burge notes that the unforgivable sin refers to complete apostasy, that is, a complete rebellion against God, the Son, and the Holy Spirit. He says, “this sin does not refer to a particular act for which one may later feel regret, but instead describes a blatant hostility to God and a serious rejection of Jesus after one has been exposed to the knowledge of the truth.”
 G.M. Burge. “Sin, Unpardonable.” In Evangelical Dictionary of Theology. Walter Elwell ed. Grand Rapids: Baker Academic, 2001. Pg. 1108.