Thre greatest act of terrorism based on religion had to be the Crusades. For 400 years or more, the Catholic chruch sanctioned the systematic slaughter and terrorizing of the Muslims in Jerusalem for the sake of "protecting" the "holy land". 400 years of war over a strip of land and an ideal against a group that was meerly living in the space that another group from thousands of miles away thought belonged to them for some reason.
In modern society, I think Fundamental Christians are the largest terrorist group, they may not be a collective, but the groups generally have the same ideals and misconstrued notions of this country. There is a collective ideal to turn the nation into this religious ideal that each group thinks the nation was founded upon which is factually untrue. The US was founded by Deists, not Chrsitains. There is a HUGE fundamental difference between the two, one follows a regious ideal and the other follows a personal spiritual path. An example of this kind of terrorism would be the Quiverfull movement which avidly rails against higher education, schooling for girls, any use of family planning, or women's rights in anyway while avowing to be the best way to god and vowing that science is out to destroy the world.
Another example would the FLDS groups which seek to destory the country from the inside out by having as many children as possible and "milk" the system of welfare. This is a group that believes that marriage to many women, even against their will is the only way to heaven and that they must have as many children as possible. Children don't go to school and women aren't allowed many rights if any.
West Bourogh Baptist is also a group one can call a terrorist group given their systematic attacks on LGBTQ and soldier's funerals as well as many famous people such as Vince Gill and Foo Fighters for the sake of scaring people into the arms of "god".
For all these groups, religion is the soul of why they do what they do and why they torture the minds of those around them with their false and horrible ideals.
http://www.history.com/topics/crusades
https://www.splcenter.org/fighting-hate/extremist-files/group/fundamentalist-church-jesus-christ-latter-day-saints
http://gawker.com/quiverfull-of-shit-a-guide-to-the-duggars-scary-brand-1706557073
https://www.splcenter.org/fighting-hate/extremist-files/group/westboro-baptist-church
