The original quote was from Karl Marx and written in German, but in English it is usually said like this: "Religion is the opiate of the masses". What is means is that religion is used as a tool to control and manipulate people and it keeps people from questioning things. Opium (or opiates) are types of drugs that make people numb. And the word "masses" in this sentence is used to mean all people. I hope this helps!

Following a religion is like a drug addict, making you numb, stupid and preoccupied with senseless rituals - allowing those in charge to control you.