For centuries, many Christians have taught that suicide is an unpardonable sin. The Catholic Church maintains that teaching to this day. But is it Biblical?
Here are a couple of passages cited as justification:
(1 Corinthians 6:19-20 NIV)  Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own;  you were bought at a price. Therefore honor God with your body.
(1 Corinthians 3:16-17 NIV)  Don't you know that you yourselves are God's temple and that God's Spirit lives in you?  If anyone destroys God's temple, God will destroy him; for God's temple is sacred, and you are that temple.
I’m not sure this proves suicide is enough to send a person to hell though. The point about the body being God’s temple is well-taken. But presumably other sins would represent a defilement of God’s temple as well. Adultery is a good example. Yet in the Bible adultery does not automatically consign a person to eternal damnation. David whom the Bible referred to as a “man after God’s heart” famously committed adultery with Bathsheba, yet we are not led to believe he is in hell. In fact, he is celebrated as a hero of faith.
Another argument for sending suicides to hell is that the person can’t repent. But if we apply Biblical standards for sin, then it’s clear people sin many times over without realizing it. They are jealous, look at women to lust after them, and harbor grudges. Would God send someone to hell who’s given his life over to Christ and tries to live a moral life because he fails to repent for every sin he’s committed?
Christianity is based on salvation by grace. That is, God can allow the worst sinners into heaven including murderers and thieves. If God is willing to be merciful to someone who spent their life killing but one day repented, are we really to believe that he would have no such compassion for someone who committed suicide to escape a life of misery and suffering? How about someone dying of an awful illness which makes him feel pain every day but with the prospect of several terrible months to live? Would a loving God really expect them to continue suffering every day until they die?
None of this is to say that suicide wouldn’t be a very serious sin. One could reasonably compare it to murder or killing. But again, even those sins aren’t enough to automatically send a person to hell. Consider the following example: a good, churchgoing man who believes in Christ. When his daughter is raped and murdered, he shoots the murderer in anger upon seeing him. As soon as the police see him with a gun, they shoot him, and he also dies. He has committed murder, yet most of us would expect God to be merciful in the above situation even if he hasn’t had the opportunity to say a prayer asking forgiveness for this specific sin.