First, I forgot to add your quote in my last post so thanks for finding my post and responding. Sorry about that.
Second, Calvinists believe OSAS because our salvation is a work of God alone, based on His grace and the work of Christ (life, death, and resurrection) alone. Salvation is not based on my work, not even .01%. I'm already guilty, how can I become un-guilty? I can't. I need Jesus to pay for my sin and be righteous for me. You see, Jesus didn't just die for us. He lived a perfect life for us because we are guilty and worthy of condemnation.
I don't know if Lutherans believe in OSAS.
Third, I think the reason why many Protestants believe you can lose your salvation is that they have a distorted view of sin and salvation. They seem to think that sin is just outward. They fail to see that when a man looks at a woman lustfully, he commits adultery in his heart (
Matt 5:27-30). Even the look is damnable. "It's OK to look, but not touch," is not true in the eyes of God. Furthermore, when he looks at her lustfully, he's only acting out the sin within. We can't be righteous on our own because sin still resides in us. No amount of good works on our part will save us. Our sin will always outdo our good works.
Concerning salvation, many think God saved us because we "made a decision for Jesus." That puts salvation in our hands. If we can make a decision for Jesus, certainly we can un-make that decision, so the thinking goes. However, if God were dependent on us to make a decision, none of us would be saved (
Rom 8:7).
God doesn't do most of the work (providing the way), and we do the rest (decide to go that way). God does all the work in our salvation. Jesus died in our place, taking our sin on Himself. He lived a perfect life on our behalf because we can't. When we were spiritually dead, God made us alive with Christ (
Eph 2:1-10). God even causes us to will and work for His pleasure (
Ez 36:26-27;
Phil 2:12-13). Our salvation is 100% the work of God. That's why we can't lose our salvation.