The moment that the Fire King's connection to his god was broken, the Shadow quickly used his Divine Sense to search his soul. The so-called True Gods were nothing but Supernatural Realm cultivators, as such, with the Shadow's cultivation at the Divine Body Realm, it was quite easy to search this guy's soul.
After doing that, the Shadow quickly returned to the kitchen and pretended nothing happened; to the outside world, he was nothing but a regular mortal.
As for the Fire King, he woke up long after that. And the first thing he did was to quickly bury these thoughts deep in his mind once again, then quickly pray to the Fire God and offer his allegiance.
The Fire King knew that his momentary lapse in faith would eventually be discovered by the Fire God, thus making it even harder for him to be acknowledged for his service and granted the opportunity to ascend to higher levels of godhood.