All Conflict is “Religious”
Short answer: Cuz they feel like they’re being attacked!
Which just raises another question: Why do they feel they’re being attacked?
Religion lives in the heart of every single human being.
I don't mean that everyone identifies with a particular church or religious institution. I'm talking about personal spirituality. Our personal identity is wrapped around our spiritual beliefs or assumptions—concerning why the world and its creatures exist; the basis of right and wrong; the purpose of life.
These are profoundly religious issues. People build their lives on the foundation of their beliefs or assumptions about these things. I keep saying "or assumptions" because often such ideas lurk below the level of conscious awareness; they are metaphysical presuppositions. Most of the time people don't think about their presuppositions unless someone challenges them.
And when such challenges arise, you're suddenly, unexpectedly ripped out of your comfort zone. That can easily feel like an assault on your very identity. As if someone is threatening the world you live in.
Nobody likes feeling that way, and it'll be worse or better depending on how much confidence in your worldview you started out with. In addition, even if your convictions are rock-solid, you might construe the other person's challenge as an insult (whether it is or not). And who likes being insulted?
And because everybody’s religious—there are literally no exceptions to this—we can’t just say, “Well, let’s get rid of religion, and then nobody will get defensive or go to war over it.”
Religion itself isn't usually the cause of conflict
On a spectrum ranging from hostile to loving, the middle zone is by far the broadest: belief-systems that don't demand conflict, but do provide grounds for rationalizing it. In this expansive category we find faiths like animism, Hinduism, Buddhism, and atheism.
Your first question might be: “Why do you call atheism a ‘faith’? Atheists don't believe in the supernatural.” The second sentence is true—but atheism is a type of faith because atheists believe in the supremacy of nature, which isn't a scientific position, but a philosophical one.
Likewise, animism, Hinduism, and Buddhism hold to nature as the “Ground of All Being.” And here's the take-away as far as human conflict is concerned: whatever you view as your “GOAB” is naturally and logically going to be the basis of your morality. Another way of saying it is: Your morality can't be greater than your maker.
When it comes to the ethics of human relationships, what these faiths all have in common is the lack of an Objective Moral Authority. In all of them the “Highest Power” is raw nature, which possesses no morality at all. This applies even to Hinduism, which has a veneer of supernaturalism but, at the end of the day, really just deifies nature itself1—so it’s still nature at the “basement” level.
The source of conflict—and peace
So, even if we could snap our fingers and suddenly live in a world without Islam—there’d still be conflict among us. The Bible spells out why:
That’s it. That’s the answer. Conflict arises when people want things and assume they have a “right” to take what they want from others. This can apply to material things like money or land, as well as emotional gratification, which is more abstract and subtle, but can be, over time, just as destructive.
Even in an aggressive religion like Islam, the problem is Self. The god of Muhammad—I’m fully convinced that “god” is a demon—persuaded him that the only way to transform the world was by force. Apparently “Allah” doesn’t have the power to change the world himself. Aggression and oppression are therefore manifestations of insecurity on the part of Muslims.
Same as the insecurity of a person who reacts defensively when you criticize his beliefs.
By contrast, Jesus invites us to experience peace—first with God, then within ourselves and with others. You can’t have a proper, maximal relationship with another human being until you are first at peace with God and with yourself.2
It starts with you and your Creator. “If your thinking is controlled by your sinful self, there is spiritual death. But if your thinking is controlled by the Spirit [of Christ], there is life and peace.” (Romans 8:6)
When you’re operating consistently under God’s guidance and empowerment, you—
“do good to everyone” (Galatians 6:10);
“as far as it depends on you, live at peace with everyone.” (Romans 12:18)
Hinduism broadly equates the universe with “God” or “gods.” It doesn't matter what end you approach it from: “the universe is God,” “gods are manifestations of the Cosmos”; whatever. Hinduism is just a glorified version of naturalism.
Obviously these overlap: as you experience more of one, it will benefit the others.