Freud was an atheist who believed that religion is:

1) obsessional -- it is the obsessional neurosis of the young human race.  We all pass through a mild obsessional stage when we are young.  Children must be socialized before they understand the reasons for socialization, so parents tend to simply repress them through guilt. Obsessional personality characteristics result from this -- ritual, repetitive behavior results as a way of anchoring the free-floating guilt produced by your socialization.  ("step on a crack, break your mother's back" is one example used in class; religion, Freud believed, is another).

2) infantile -- religion is an infantile response to our ontological (deep, "of being") insecurities.  When we were young, Mom and Dad kissed our bumpies, tucked us in at night and made every bad thing better.  But when we grow up, we realize that there are some bumpies that Mom and Dad can't make better -- so we project Mom and Dad into the heavens and they become the Gods.  The Gods, of course, CAN kiss every bumpie and make all things better.

3) illusory -- an "illusion" is a belief in which wish-fulfillment is of primary importance.  Whether the belief is true or not is of only secondary importance.  Religion fulfills our infantile desires for safety and security.  It would be nice if religion were true, Freud observes, but we can neither prove nor disprove it.  Therefore, belief in religion is merely an illusion fulfilling our infantile desires for safety and security.