Romans 4:14 :
For if those who are of the law are heirs, faith is made void, and the promise is made of no effect.
What good is salvation through the death of Jesus on the cross if all you had to do was to follow a set of rules and earn your righteousness and peace with God? When a person decides to earn their way to God, they decide they can replace what Jesus did. There is no way that any merit system can be included in salvation. That is what the Bible clearly says here.
This concept challenges all other religions because, as far as I am aware, all other religions are merit-based. They condition your good relationship with God (or the gods) on your behavior. I am not saying that there are no merit-based blessings in Christianity. What I am saying is that a Christian’s salvation from punishment for breaking the laws of God can only be based on faith in Jesus. Any other form of “Christianity” is not really Christianity. This also demonstrates why Christianity cannot be joined with any other religion. Any kind or religion that involves a merit-based way of gaining favor with God destroys Christianity’s “grace only” way in the process.
There is no middle ground. Either a person is a saint or that person is not a saint. There is no way that God has provided to earn our way back to Him other than the law and we can’t do that.