There are a lot of people who struggle with doubt, and there are several reasons.
Firstly, some people don’t know enough about the doctrine of salvation to understand that it gives built-in security, so they need a more biblical understanding of salvation. There are a lot of people who minimize doctrine, but the very foundation of a believer’s security is a full understanding of the doctrine of salvation. You can dismiss penal substitution, or you can believe it and have the foundation laid for understanding that your salvation is eternal.
Secondly, I think people doubt their salvation because they are in a situation where doubt is cast on salvation as if to say, “You could possibly lose this.” The reverse of that is this: don’t go back to an event and say, “I know I’m a Christian because I prayed this prayer.” Ask simple questions like: “What do I love? What do I have strong affections for?”
If you’ve been transformed by the power of the Spirit of God, if you are a new creation in Christ, there are going to be some marks. I’ll lay these out in the simplest way I can.
The first mark is love. You will love the Lord. There will be strong impulses in your heart toward Christ, toward the Lord. You will love the Scriptures—that’s not an unbeliever’s affection. You will love the church. You’ll want to be there. You will love God’s people, and you will even love the lost.
The second mark is humility. If you’re a true believer, you’ll be marked by humility. You won’t be proud, boastful, brash, or self-exalting. There will be just a very sweet kind of brokenness that’s just part of you.
The third mark is that you will desire what is right. Your heart will long for those things. So, it’s not the perfection of your life that demonstrates your salvation. It’s the direction, and it’s in the direction of love, humility, and righteousness.