Explain what Marx meant by the idea that religion is the opium of the masses.

What Karl Marx is saying here is that religion is like a
drug.  Specifically, it is like a drug that keeps people happy so that they do not want
to rebel against the oppressive governments and societies that hold them
down.

Marx believed that all people in the oppressed
classes would inevitably rebel against those who held power in their society.  Peasants
would do this to feudal lords, workers would do this to the bourgeoisie.  But this often
did not happen and workers, for example, often showed no desire to rebel.  Marx had to
explain this.

One way in which Marx explained this was to
say that religion helped to keep the people down.  They would focus on the lives of
their souls instead of material things.  They would disregard their problems on earth
and focus on the idea that they would be rewarded in the next life.  In this way,
religion kept them from wanting to rebel.  It kept them focused on something other than
the way they were being oppressed.

This is why religion
was, for Marx, the opium of the masses.