Two friends recently asked me about this, so I thought it might be good to address. But I always want to be careful as I never know who might read this, and what people are going through or have gone through. So I pray you only hear biblical truth here to encourage your souls toward Christ. A question like this can only be answered by putting a lot of different teachings together:
1. Suicide is murder- that is the only vice that is listed in the Bible in which that would fall under; suicide is the exact same type of sin as homicide, abortion, and hatred in your heart (Genesis 9, Matthew 5, 1 Tim 1). All those actions are violence toward someone who is made in the image of God.
2. Blasphemy of the Spirit is the only unforgivable sin- to commit this sin, one must charge Jesus with serving the Devil (Matthew 12.22-32). Christians cannot– meaning do not have the ability– to commit this sin.
3. No one can lose their salvation- Every single person God saves is someone He foreknew, predestined, called, justified, and one day will glorify (Romans 8.28-30). Anyone who falls away from the faith only proves that they were never born again to begin with (1 John 2.19)
4. There is no verse that teaches suicide is the blasphemy of the Spirit.
5. Christians can do heinous things- I have. I have committed murder in my heart. Christians cannot blaspheme the Spirit, but they can pretty much still do anything else, given the way sin and temptation and suffering work on this side of heaven.
6. Christians must not commit suicide in the same sense that Christians must not lie, steal, murder, or commit adultery, or any other command of the Lord. Any sin can and will be forgiven (1 John 1.9), and it is only willful, unrepentant sin that should make us question whether someone is born again (Matthew 18.15-17)
7. Christians normally only want to please God.
8. We have much more victory in the Christian life than most Christians ever realize (Romans 6-8). So the temptation and the act of suicide (and any murder for that matter) should always at least make us wonder how much someone really knew our Savior.
My conclusion: suicide is not a damning sin. I do think it is near damning evidence that someone was never born again, but that is as far as I think the Bible goes.