Karl Marx called religion the opium of the masses.
The belief that Jesus would return again and right all wrongs has helped many Christians throughout the centuries to endure oppression and unjust treatment.
Marx thought such beliefs, such illusions about the future to be an impediment to political and economic progress. While people remained high off of the drug of religion, they would not try to eradicate the source of their pain and suffering.
Not long after the fall of the USSR, the Nobel Prize winning poet from Poland Czesław Miłosz wrote about the true opium of our age:
A true opium of the people is a belief in nothingness after death—the huge solace of thinking that our betrayals, greed, cowardice, murders are not going to be judged… One may wonder at the image of Ascension in the New Testament, but the vertical axis rules everywhere in the noncorporeal world.
Remove the belief in a God who takes into account everything that one has done in this life, and you are left with an open door to the worst of human behavior. (If you doubt this, remember the atrocities of recent history acted out under Stalin, Hitler, Mao Zedong and Pol Pot.)
Marx was wrong about Christianity. It is no opiate.
Quite the opposite:
To the thirsty I will give from the spring of the water of life without payment. The one who conquers will have this heritage, and I will be his God and he will be my son. But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death. (Revelation 21:6-8)