It unfairly allows itself social and political advantages that are not granted to Muslims, Jews, Buddhists, Hindus, atheists, and whoever else. It winds its way into government policy and starts influencing decisions that affect people who do not subscribe to it, forcing them to comply with its doctrines even if they don't share them. It actively tries to suppress the influence of other religions or secular belief systems in the US instead of trying to help create a tolerant society. If a Buddhist ran for president, they would never win, even if they had the best policy ideas, because Christians can't bear to see a non-Christian in the supposedly-secular office of government.