All the great song writers were mediocre singers. I’m talkin’: John Prine, Steve Goodman, Townes Van Zandt, Leonard Cohen, Paul Simon, Neil Young, even Willie Nelson I would put in this category.
A weird exception is Joni Mitchell. IMHO one of the best singers of all time, and an a-list writer too.
belief and faith are not the same thing. it’s entirely possible that you meant what you said, but I’m guessing you didn’t.