They would be willing to freely and easily discard or modify their beliefs in favor of something better. God's existence is, for them, a fact. Once again, you are attributing the beliefs of a small number of people, those with narrow, fundamentalist religious beliefs, to everyone who believes in God. You have explicitly stated that God's existence is, for you, a fact. In the final analysis, any knowledge possessed by man, be it political, economic, religious, or scientific, can be twisted to evil purposes.
Perverted science is just as harmful, dangerous and destructive as perverted religion.
As I said earlier, any good tool can be misused. Religion certainly seems to think along those lines. As far as the matter of religious strife goes, there's been plenty of that through the ages, but all of it put together is a church picnic compared to the non-religious strife of the 20th century. In fact, think of all the most dangerous hot spots in the world today. Religion plays a very minor role in any of them, except as a vehicle for evil men to whip up support for their causes.