Almost everyone I know has had doubts about their salvation at some point. I went through a period of about five years where I was certain I was destined for hell. In John 10:29, Jesus said, "My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand". This includes you. You are not more powerful than God. Once you have received the gift of salvation, it is FOREVER. If someone backslides and never turns back to the Bible or the church, they were never saved in the first place (1 John 2:19). But for those of us who have accepted Jesus as our personal savior, there is absolutely nothing we can do to blow it. This doesn't mean there might not be consequences. We can do plenty of things that affect our fellowship with God, but nothing we can do to affect our relationship. Recall the story of the Prodigal Son. The son was always the son, even when he was away from home and living in sin. But when he repented and turned away from his sins, the fellowship with his father was restored.
Maybe someone you know is still uncertain. The best way to deal with this is to get rid of false beliefs. I suggest they read Galatians over and over until their mind is saturated with the fact that only the Holy Spirit can begin and continue the ongoing work of sanctification (Galatians 3:3).
But what about the unpardonable sin?
I struggled with blasphemous thoughts about the Holy Spirit for three years. One night, I was so angry at God about my circumstances that I stopped struggling against this thoughts and just gave in to them. It was such a terrible thought that I refuse to share it with anyone at this moment. I did not sleep at all that night because I was certain God had left me. This was a lie from the devil because Jesus said he would always be with us (Matthew 28:20) I was so sure for years that I had signed my own eternal damnation papers. But there was a catch. I was under a false belief system. The God I had conceived in my mind was not the God of the Bible. In other words, I was worshiping an idol. After leaving that false belief system behind, I started to feel God's presence again. I became happy again. I also know that God did not count those thoughts against me since I was deceived.
In Ephesians 2:8-10, Paul said:
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them.
Jesus did it all. The only unpardonable sin is refusing to accept Jesus as your savior. A truly saved person will always come back to Jesus even if it takes years.