It wasn't like they had seen it coming, hell they didn't even find out about their followers until late in the game, and it had been a fluke even then. However, even before the Holy Scriptures where published the two had a large faithful following. They just hadn't been aware of what it meant. This story doesn't start there, no this is the story of how two brothers became Gods for their modern world.
It had been happening slowly for years, they didn't really have much to compare their lives to so they had just assumed that everyone healed as quickly as they did. Everyone knew the Winchester family had enough luck to power a nuclear reactor so they also didn't question when things that should have been impossible happen. Their lighters never needed to be filled, their guns never failed to fire, unless turned on them. They would stumble onto a gas station just as the impala was about to run out of gas. Rock salt was always available even in the southern states. The list runs on and on.
It was after heaven was defeated, and Cas was gone that they found out the truth. The leviathans wanted them dead, and it made sense after all they had killed Gods, Angels, Demons and everything else that crossed their paths but that wasn't the reason why. Dick Roman knew what they were even before they did, they were modern Gods, followers spanning the entire world and gaining more every day. People they saved, fellow hunters, readers of their story, creatures that whispered their name in the shadows. No, Sam and Dean weren't just hunters anymore, they were Gods and Gods good or bad are dangerous.