He goes on to speak of those who continue believing, and are therefore not destroyed. A person who thinks a believer cannot quit believing has not thought the matter out. He did not promise salvation to such people, and woe be to the person who represents Him as saying that. Such people will surely be saved. God will underwrite their efforts.
Faith is the foundation of our salvation.
By that I mean believing and resting in what Jesus has done is the basis for us being saved. We can contribute nothing to that foundation. God saves us because of what Jesus has done, and because we are trusting in that. He only says he does. The acid test of real faith is being able to do what God commands.