As noted the Muslim population is the second largest behind Christian but even then it does not begin to touch the Christian population
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Of course, these statistics in and of themselves do not tell the story.  All they point out is that in a nation of predominant Christians, the predominant religion in prison is Christian.
 
That is not surprising.  What would be more interesting is to compare the stated preferences of prisoners with the population of the US as a whole, and then run a T-test on the percentages to see if there is a significant difference between the stated percentages.
 
A T-test compares the percentages and looks for statistically significant variations in the data.  That would actually tell if there is a difference.