Federal Bureau Prison population percentage by religion:

http://wp.production.patheos.com/blogs/friendlyatheist/files/2013/07/rNyTbDJ.png

Percentage of US population who are atheist:

http://www.pewresearch.org/fact-tank/2016/06/01/10-facts-about-atheists/


In summary:

Atheists comprise 0.2% of the federal prison population.

Protestant and Catholics alone make up 75% of the federal prison population, which doesn't include the other Christian religions.

About 7% of the U.S. population are atheist.
About 70% of the U.S. population is Christian.

Conclusion:  By the actual numbers, you're much more likely to have weak morals if you're a Christian.