Apparently the Atheists population in prisons is less than 1% (0.2 %) to be exact. This is not saying that being religious will lead you to crime but could mean that being religious will not PREVENT you from committing crimes. Basically religious people are not any better/moral than non-believers.

Why do atheists comprise of less than 1% of the prison population?

1) Atheists are more intelligent AND just as moral and are less likely to commit crimes?

2) Atheists are generally more intelligent and less likely to get caught committing crimes?

3) Prisoners convert to Christianity or other types of religion AFTER committing the crime WHILE in jail ? Therefor these statistics could be incorrect?

According to CIA.gov (the link is below) , About 78% of the US population are religious. This means that there's a higher chance that a person that committed a crime will be religious NOT atheist. This is basic common sense. So the reason why there's very few atheist in prison is because there's just so many more Christians/religious people in general. This doesn't mean that being religious will mean you are more likely to commit a crime but means that being religious does not mean you have better morals than that of Atheists or nonbelievers. This is a fact.

Sources : http://atheistempire.com/reference/stats/main.html
https://www.cia.gov/library/publications/the-world-factbook/geos/us.html