The Nords have worshipped dragons for thousands of years, beginning long before the events of The Elder Scrolls V: Skyrim. The dragon cult began with when Alduin, the World Eater, led dragonkind to conquer and subjugate the early peoples of Tamriel.
Though not all Nords chose to worship dragons, Dragon priests appeared and the cult grew. Temples were built all through Skyrim.
It only after Paarthurnax taught the Thu'um to mortals that they rose up to challenge Alduin and put an end to the cult.