Suicide is wrong and a sin.
But only unbelief in Christ as redeemer is unpardonable and leads to eternal death.
I will assume you are speaking of “Once saved always saved”
St. James, in his epistle, gives the example of an erring believer and how he can be brought back: “Brethren, if any of you do err from the truth, and one convert him; Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins” (James 5:19-20). “Any of you”! Note again that he is talking to people who are of the flock. If a Christian errs from the truth and is converted, the good man who helps him reaps these spiritual benefits.
James was speaking of someone already in Christ. So, why would a person already saved need to be saved from death, if once save always saved?
Seems I’ve read that somewhere...