Why incredibly unbelievable?

I heard it again today from an Indianapolis pastor online. He was excited about the Gospel and called it "unbelievable." Incredible. No! It's believable! Credible! Instead, let's use Awesome! Awe-inspiring! Phenomenal! 
                                                                                                                                                         

breathtaking, magnificent, wonderful, amazing, stunning, staggering, imposing, stirring, impressive; formidable,  mind-boggling, mind-blowing, jaw-dropping,  excellent, marvelous;  literarywondrous;  "the Grand Canyon is as awesome as they say it is"