In 1844, Marx lived in a time when almost everyone went to church and religion had a strong influence on every level of society. Today, religion plays little or no part in the lives of hundreds of millions of people.
He claimed that mankinds needs could best be met by materialism, a philosophy, by the way that leaves no room for God. He stated,
"The first requisite for the people's happiness is the abolition of religion."
It should be noted that the Churches must share the blame for inhibiting social change.
For centuries they have abused their authority. And they teach doctrines based on unscriptural traditions and human philosophies rather than the Bible.
Evidence?
During the 1960's, Protestantism came up with a theology that proclaimed the "death of God." Many Protestant clergymen condoned a materialistic life-style. They did not denounce premarital sex or homosexuality.
Some Catholic theologians developed liberation theology, mixing Catholicism with Marxism.
French sociologist and philosopher Edgar Morin admitted:"Not only have we seen the collapse of the brilliant future held out to the proletariat but we have ALSO seen the
collapse of the automatic and natural progress of secular society, wherein science, reason, and democracy were supposed to advance automatically...No progress is now assured. The future we had hoped for has collapsed."
Such is the empty feeling of many who put their faith in man's efforts to create a better world without God.
The complete quote from Marx is,
"Religion is the sigh of the oppressed creature, the sentiment of a heartless world, and the soul of soulless condition. It is the opium of the people."