Via Dan Savage, a meta-analysis showing a correlation (not necessarily causation) between religious dogmatism and racism:
The February issue of Personality and Social Psychology Review has published a meta-analysis of 55 independent studies conducted in the United States which considers surveys of over 20,000 mostly Christian participants. Religious congregations generally express more prejudiced views towards other races. Furthermore, the more devout the community, the greater the racism.
This study finds that a denomination's demand for devout allegiance to its Christian creed overrides any humanistic message. By demanding such devotion to one specific and dogmatic Christianity, a denomination only encourages its members to view outsiders as less worthy.
Moreover, the study found that agnosticism correlates with tolerance, to which I think one should add "Q.E.D."
Again, the study doesn't show causation, only correlation. Religion doesn't itself make one racist. Possibly the conditions that lead someone to religious dogmatism also lead to racism; possibly the communities in which more-devoted religionists live are in areas with historically higher racism.