Karl Marx called religion the "opiate of the masses." If you are a person of faith, then when push comes to shove you are less likely to run to the government for your peace of mind. Karl Marx believed that, and so do today's Democrats.
The Democrat Party booted God from their platform a couple presidential elections ago, only to add Him back after heavy criticism, despite the chorus of boos coming from about half of the lefties in the auditorium at the time.
When it comes to the Democrat Party, however, it isn't really religion that bothers them. They are fine with Islam, Buddhism, and anyone who claims to be religious but carries their ungodly agenda on their sleeves. Their problem is with practicing conservative Jews, and Christianity. The latter, in fact, is so hated, that they would be willing to outlaw it if they had the chance.