DanPhillips explains it:
What the “Islam is not a religion” crowd is doing, whether they realize it or not (and most don’t), is imposing on the definition of religion a philosophical concept that is relatively novel (historically speaking) and that potentially binds theology beforehand. Per their reasoning, in order to be a religion a religion must embrace modernist liberalism. This would have been news to anyone—Christians included—who lived, say, more than 300 years ago, give or take. One commenter I was debating with said that Islam is not a religion because it doesn’t embrace separation of church and state. Really? Are we that historically myopic? Neither did the whole of Christendom until a couple of centuries ago.
By their definition of religion, the Judaism of the Old Testament was not a religion. Was not the Judaism of the Old Testament an all-encompassing system that mixed church and state, had religion-based laws, had a social order dictated by the religion, frowned on pluralism, etc.? The Catholic Church, especially before Vatican II, is not a religion by this definition. Arguably, and it would be hard to argue otherwise, the Protestantism of Luther and Calvin wasn’t a religion either. Was Calvin’s Geneva a bastion of modernist liberalism? The Puritans certainly were not. One would have to look back no further than the Radical Reformation to find widespread Christian denominations that would meet the exacting liberal standards of the “Islam is not a religion” proponents. (And even some of the products of the Radical Reformation, such as the Mennonites, were quite illiberal in many ways internally.)
I hope you see the problem here. I would argue that liberalism is a modern philosophical concept that most modern Christians have read into the pages of the Bible (addressing this idea fully would require a separate essay). I do not think this liberalism is a theological concept that flows from a natural reading of Scripture. The Bible insinuates, if it doesn’t outright dictate, Christian particularism. Christianity should be the broadly encompassing worldview that Islam is accused of being (in type, not in detail of course) and it represents a failure of the modern Church that it is not.
This idea that Islam is incompatible with America and the West (what used to be called Christendom) because it is illiberal, implies that what truly distinguishes the West from the rest is its liberalism not its Christianity. This may be true and would go a long way toward explaining the sorry state of modern Christianity, but it is to be bemoaned if it is, not celebrated.