You assume a truth and a God. So do I. God gave us truth with regard to good and evil. We need to be rational in living our lives according to that morality. But that morality did not emanate from the use of reason. If so, all rational people would be good. They are not. Many rational people (Dr. Moriarity, the Grinch, Lex Luther, Stalin, Hitler, Pol Pot) are perfectly evil.
I make no such assumptions. My belief in God is the result of a very long and arduous chain of reason. Truth is not a thing to be "believed" or "not believed," it is a quality of a certain class of statements.
For your information, Ayn Rand might have considered the existense of God, except that Christians convinced her it was a mistake. There were certain metaphysical questions which she admitted the "supernatural" could solve. She once said, she felt physics had abondoned the idea of the "ether," (a less than purely physical concept) a little too soon.
You also did not answer my question. If reason is not the means by which I know what is good and bad, what faculty do I use?
(You made an earlier absurd statement about knowing what your desires are without reason. You used language to describe the desire, and language is a "rational" process, that is, a function of reason. When you have a desire, how do you identify it, how do you decide whether it is the right time to fulfill it, how do you know it is not just indigestion?)
Maybe you just live by whim and impulse.
Hank