Suicide is a sin because it is self-murder. Nowhere does the Bible say it is the unpardonable sin though. To say that suicide is the unpardonable sin is unbiblical. Samson committed suicide. “Then Samson called to the Lord, saying, ‘O Lord God, remember me, I pray! Strengthen me, I pray, just this once, O God, that I may with one blow take vengeance on the Philistines for my two eyes!’ And Samson took hold of the two middle pillars which supported the temple, and he braced himself against them, one on his right and the other on his left. Then Samson said, ‘Let me die with the Philistines!’ And he pushed with all his might, and the temple fell on the lords and all the people who were in it. So the dead that he killed at his death were more than he had killed in his life” (Judges 11:28-30 NKJV). Samson is listed among the heroes of the faith despite this act at the end of his life that killed both his enemies and himself. “And what more shall I say? For the time would fail me to tell of Gideon and Barak and Samson and Jephthah, also of David and Samuel and the prophets: who through faith subdued kingdoms, worked righteousness, obtained promises, stopped the mouths of lions, quenched the violence of fire, escaped the edge of the sword, out of weakness were made strong, became valiant in battle, turned to flight the armies of the aliens” (Hebrews 11:32-34a NKJV). Samson would not be included among the righteous if his final act of suicide condemned him.
There is the only place where we see a reference to someone not being able to be forgiven. The religious leaders of Jesus’ day were attributing the miraculous works He was performing through the power of the Holy Spirit to an evil motive. Jesus said the only sin that could never be forgiven is blasphemy against the Holy Spirit. Blasphemy against the Holy Spirit is the result of continual rejection of the Holy Spirit’s efforts to convert you. “Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come” (Matthew 12:31-32 NKJV).
Those who have turned from their sins and trusted in Jesus alone for forgiveness are not condemned. “There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit” (Romans 8:1 NKJV). All sins are removed once and for all when we come to Christ in humility and repentance. “All who sincerely trust in Him are made right with God. For Christ is the end of the law for righteousness to everyone who believes” (Romans 10:4 NKJV). A person who commits suicide and is a genuine follower of Christ has committed a horrible sin by killing themselves. They will probably lose eternal rewards because of the suicide. The issue with suicide is not one of eternal destination however. Jesus said, “I am the way, the truth, and the life. No one comes to the Father except through Me.” (John 14:6 NKJV). That is the only determination of eternal destination. God says that nothing can separate one who belongs to Jesus from His love. The Apostle Paul wrote, “For I am persuaded that neither death nor life, nor angels nor principalities nor powers, nor things present nor things to come, nor height nor depth, nor any other created thing, shall be able to separate us from the love of God which is in Christ Jesus our Lord” (Romans 8:38-39 NKJV). Only people who are not true Christians–those who have not genuinely repented of their sins and trusted in Jesus for salvation–will be separated from God and all good things that come from Him for eternity in Hell. “He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, but the wrath of God abides on him” (John 3:36 NKJV).