There is no sin a Christian can commit that God cannot forgive. Romans 8:1 teaches, "There is therefore now no condemnation for those who are in Christ Jesus." When Jesus died upon the cross, His sacrifice was sufficient to cover every sin of every person who would come to faith in Him.
Are there any unforgiveable sins?
However, Jesus did once speak about an unpardonable sin during His earthly ministry. Matthew 12:31-32 shares, "Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come." The context involved Jewish leaders who had attributed the miracles of Jesus to Satan. In other words, they were rejecting Christ as the One sent from God as Lord and Messiah.
Some believe Jesus spoke these words only to those in His audience and that this "unpardonable sin" cannot be committed today. Yet, even if this is true, there is a sense is which the principle remains. The one sin God will not forgive is the person who rejects Jesus Christ. Jesus said He was the way, the truth, and the life and that no one can come to the Father except through Him (John 14:6). The apostles likewise taught, "there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved" (Acts 4:12).
Another important verse regarding this question is found in 1 John 1:9. There we find, "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." It is important to understand that this verse was written by the apostle John to a group of believers in Jesus. Christians still sometimes sin. When we do, we are called to confess our sins and be forgiven and cleansed. However, it is also important to understand that those who have placed their faith in Christ have already become children of God and are loved and forgiven. This truth does not offer a license to sin (Romans 6), but rather a confidence that we can know we will have eternal life (1 John 5:13).
Jesus did not come to condemn the world, but to save people from their sins: "For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God" (John 3:16-18). Anyone who believes in Christ will be saved and forgiven. It is only the one who rejects Jesus as God's risen Son who will not be forgiven and will forfeit eternal life.
Does God forgive big sin? Will God forgive things like murder?
How do I receive forgiveness from God?
Blasphemy against the Holy Spirit - What is it?
Is a death bed conversion possible?
Does the Bible teach eternal security?
Truth about Salvation