When I was much younger, I was practicing a particular memory verse that got me thinking and actually bothered me, 1 John 5:13 – “These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.” It bothered me because I didn’t know, and I knew that it was important that I did know. I asked a lot of questions and eventually became saved. This became my favorite verse.
If you don’t know Jesus, or if you do know Jesus and you’re having doubts, know that you can know. You can know for sure about your rescue from sin and death. You can know for sure that the Holy Spirit is within you.
In 1 John 5, especially around verses 9-13, there is some rich explanation about how we can know. Also take a look at 1 John 5:6 and Romans 8:14-17, and their surrounding verses. When we get saved, the Holy Spirit comes to live inside us, and so tells us that we belong to God. When the Holy Spirit is in us, it makes it a whole lot less fun to sin. When the Holy Spirit is in us, it makes it more enjoyable for us to keep God’s commands.