Growing up, I was taught that suicide was the one sin that God would not forgive. The rationale behind that teaching was that the person was incapable of asking forgiveness because he or she was obviously dead after committing the sin. The Bible does indeed tell us there is a sin that God will not forgive:
Matthew 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
Blasphemy against the Holy Ghost is the unpardonable sin. What exactly is blasphemy against the Holy Ghost? When you blaspheme the Holy Ghost, you are saying that you don’t believe what the Holy Ghost said or did is true. What did the Holy Ghost say or do?
2 Peter 1:20-21 Knowing this first, that no prophecy of the scripture is of any private interpretation. 21For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
The Holy Ghost gave us the scriptures – the Bible. When you say you don’t believe the Bible – that you don’t believe what God said about his Son – you are calling God a liar.
1 John 5:10 …he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.
That is a serious charge to make. When you call God a liar – when you don’t believe – that is the sin of unbelief and you are blaspheming the Holy Ghost. You will not be forgiven for that.
Romans 3:4 God forbid: yea, let God be true, but every man a liar.
John 3:18 …he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.