I didn't lose my salvation, because even though I sinned more than what I thought God would forgive, I kept the faith, I kept trusting in Jesus Christ, and I'm still saved. I've been growing constantly since I became a Christian.
But a friend who was raised a Christian, and who I know was once saved, as in having the Holy Spirit and saving faith, lost her salvation, because she left, and she isn't coming back. I saw her fall further and further away from God, and then she married an atheist, and there's no light in her now. And I asked God in prayer, and eventually got the answer that she isn't coming back. Because it's about faith. She rejected Truth, because she didn't want to live according to His commandments.
I don't know a lot about theology, but I do know Scripture, and it says that only those who persevere until the end will be saved. And even many who believe that they are saved, aren't.
But for someone who is truly living for God, like I know I am, I know He would never let me go. He doesn't allow us to fall away, nothing can snatch us out of His hands or separate us from His love. Unless we start denying Him, and then keep denying Him, then He will deny us. 2 Timothy 2:12 Peter came back to Christ, but Judas didn't.