The Undead Temple would casually send out a few priests, mostly Silver Skeletons. Just think about it, how would those clueless Silver Skeletons go about gaining followers? Hence, the Undead Godhood has always been rather obscure.
But no matter how obscure it is, it still belongs to the King. Who do you think you are, some random nobody who just stumbled in here, daring to call yourself the Undead God?
If those extremists from the Church of Light heard you babbling about the Gods of Light, they would tear you apart on the spot. However, Harvey and the Great Sage are not so extreme. They just burst into laughter:
"So you are the Undead God? That's great. Look at my hand. This is the Fire of Mass Faith. If your master is the Undead God, he should know who I am. Please ask him to help me extinguish these flames. If he can do it, then I will believe that he is the Undead God," Harvey laughed.