Why faith alone? The answer...

[Debp]

Just as you freely accepted Jesus in your life you can also freely choose to stop serving him. Jesus said with his own mouth; (Matt. 24:13) But he that shall endure unto the end, the same shall be saved. That is until the end of your life, or until the Second Coming of the Lord.

That means because we are saved, we will endure. If someone isn't born again, they won't endure.