How can a 12 to 14th Century way of thinking exist in the 21st century?

Saudi Arabia helps in the fight against ISIS while there are those among the Saudis that are partly responsible for its existence and for radical Sunni Islam.
Tolerance for Saudi Arabia’s conflicted behavior is from its holding great wealth from petroleum.
As dependence on foreign oil wanes in view of nations pursuing economic sustainability based on renewable energy, tolerance for Saudi and other Arab nation transgressions is growing short. Alliances must be considered short-term and for convenience and necessity.
The news is that Saudi Arabia has issued a death sentence for a Shia leader. Muslim sects are opposed to one another and that conflict is manifest in the war in Iraq, Syria and most everywhere else in the Middle East. It is fundamentally about the absence of tolerance in the Islamic faith. While some might argue that tolerance is possible, the evidence demonstrates otherwise.
That is why the free world is said to be at war with Islam. Unless and until the faith amends its beliefs such that it can coexist peacefully in the world, the religion loses its status as such to become nothing more than a terroristic belief system bent on the destruction of humanity. For that, there will be no tolerance.
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Official hatred: Saudis confirm disdain for Shiites

 

The answer to this question in the title is: "It can still exist in the middle east somehow but likely millions are now going to die who are Sunni and Shiite Muslim because of it.

There have been religious wars in the past for example, the war between extremist Hindus and extremists Muslims created, for example, the Sikh religion which worships Hinduism, Islam, Christianity and even Tibetan Buddhism and other Buddhist ideas in a very progressive way. This Sikh religion if you study it's history came out of unending violence between Hindus and Muslims in India for centuries at a time.

Out of the millions of potential deaths now from a war between Shia and Sunni Muslim peoples hopefully will come some sort of peaceful future for both sects and for their relationship with Christianity and Hindus and Buddhists and others around the world. 

If not, many more millions will die around the world from the intolerance inherent in both Muslim sects still today.