AtheismWonder and Relic victories take +100 years-50% Spies/Treason cost

"[Player name] mocks your puny achievements. Relic and Wonder victories will take longer."

Message when an enemy has researched Atheism

The effect regarding Wonders and Relics affects all civilizations, not just the player who researches it.

Choosing to research Atheism is entirely based on the situation of the game, and is less effective in games where players are opting to win through conquest. In games where players are attempting a Wonder or Relic victory or in the Defend the Wonder game mode, the extra hundred years combined with their special ability to quickly raze enemy structures makes it considerably easier for the Huns to spoil their opponents' attempts for a Wonder or Relic victory.

Secondly, the significant cost reduction for Spies greatly benefits the Huns in their ability to scan enemy bases and observe their activity, especially since most players from other civilizations would likely avoid researching Spies due to its profoundly steep cost (200 gold per enemy Villager).

"The paganism or outright lack of faith of the Huns offended the Romans who came to think of them as truly sub-human. The Huns, in turn, were unencumbered by the laws of structured religions. Laws of social and religious order were too restrictive for a civilization that was constantly on the move. The Huns put little faith in monuments and artifacts, therefore, focusing mainly on the here and now."

See more here:

Atheism | Age of Empires Series Wiki | Fandom