That is very true and I agree that a religious America has not been the threat to the world that the Communists and the Nazis were. Not even close (despite what Nancy Pelosi believes.)
So Prager is correct in that simple comparison. But look again at what he says:
Secular fervor, i.e., communism and Nazism, slaughtered, tortured and enslaved more people in 50 years than all Europe's religious wars did in the course of centuries.
That is way to broad a generalization.
He is equating being nonreligious (i.e. secular) to Communism and Nazism. That is simply not true and by defining these aggressive socialist dictatorships by the single parameter of their nonreligious or even anti-religious foundations is way too much of an oversimplification for me. The world just doesn't revolve around whether you are religious or not. There are other things and Prager dismisses them all.
Now that doesn't make Cohen right either but I'm picking on Prager not Cohen.
Prager’s point is quite correct in that Nazism and Communism have never arisen in a Judeo-Christian nation. And that hasn’t happened because the foundations of those systems is in conflict with Judeo-Christian beliefs.
Have you read Marx? If so, how else can you consider the flavor of Communism he birthed anything but secular?