Complex sentence: Arguably, the greatest difference between Christian and secular libertarians concerns those who are not only libert "arian", but also libert "ine"--that is, they want to do the very things which Christianity forbids.
Split into simpler sentences: Arguably, the greatest difference between Christian and secular libertarians concerns those who are not only libert "arian", but libert "ine". That is, they want to do the very things which Christianity forbids.