Salvation and Eternal Security
Regarding the specific question concerning a loved one’s eternal security, my response, in summary, usually sounds something like the following.
There is nothing in the Bible that ever indicates that a believer in Christ can ever lose their salvation, their eternal security. The Apostle Paul is clear that there is now therefore no condemnation to those who are in Christ Jesus (Romans 8:1). Paul continues in that chapter to state that nothing can separate us from the love of God in Christ, including death—by any means (Romans 8:28-39).
The “Unpardonable Sin”
Some ask whether suicide might be the “unpardonable sin.” The only unpardonable sin is to willfully and permanently reject God’s offer of salvation through Jesus Christ (John 3:36).
Jesus Christ’s sacrificial death atones for all the sins of his people—past, present, and future (Romans 3:25). Believers in Christ enjoy God’s enduring and complete forgiveness for all their sins (2 Corinthians 5:18-19).