Studying different theologies
If Christians read too much outside their own tradition they are likely to end off confused and uncertain which sect to believe. After all the different sects each give convincing and logical seeming reasons why their particular interpretation of "Christian revelations" has to be the one and only true interpretation and why all the other sects must be woefully wrong.
If atheists read too much theology from different sects they are likely to decide there must be something wrong with the belief systems. So many different sects find reasons why every interpretation of supposed revelation except their own is faulty. It looks like they are all faulty and atheists can find better, more interesting things to do than reading too much theology.