I do not believe a true believer in Christ would lose his salvation because he or she committed suicide. Christ’s death covered all of our sins, past, present, and future. There is no sin that the blood of the Infinite Son of God cannot cover for those who trust in Him.
Yes, I know that there is the unpardonable blasphemy against the Holy Spirit, but that sin cannot be committed by a true believer because it is the intentional and knowing rejection of God.
To be sure, suicide is a serious sin as it is a direct attack on God’s image. However, there are many sins that are direct attacks against God’s image that God pardons. Scratch many - all sins of true believers are pardoned. I think our focus should be on the Savior who covers our sin and not the sin itself.
Now I do think the suicide question is a good one and very troubling to those family members and friends who survive, because you do wonder more than anything if the person who took their life was a true believer. It is heart wrenching. It is hard to reconcile. I know that I am a true believer in Christ, but sometimes I knowingly and with evil intent sin. I find my sins heart wrenching and hard to reconcile. Isn’t my sin in the same category as one who commits suicide? But you know what, my Father takes me in His arms and loves me anyway.
I have known several people who committed suicide and others who have contemplated suicide. One taught Sunday school to the children before I would arrive to teach catechism. I believe she is in heaven. She made a bad choice, but God superintends over our bad choices. That’s what He does. We sin and He forgives. He is really good at forgiving. He has been forgiving for a long time.
My understanding that the desire to take your own life is an intense and usually brief desire that if you hold out, will pass. Isn’t that the experience of all sin? I think there may be people who struggle with the sin of suicide, just like some struggle with drug addiction or anger or pornography. All of us probably struggle with besetting sins. We just need to run to our Father. He knows us. He is there for us.