We face an external threat from radical, political Islam. That enemy—the enemy without—wants to sweep away Western civilization and impose a global caliphate.

We face an internal threat from a coalition of secular humanists, atheists, and leftists. The enemy within wants to erase the United States’ Judeo-Christian values and replace them with the false dogmas of relativism, sexual liberation, and hedonism.

The enemy without and the enemy within could not be more opposed to each other in theory. Yet the surprising alliance between these two groups goes largely unnoticed in the media. Not long ago, during the Women’s March on January 21, 2017, leftists demanded unrestricted abortions and LGBTQ rights. Oddly, however, the march organizers chose as co-chair an Islamist, Linda Sarsour. She advocates for Sharia law in America, neglecting to mention that it is incompatible with democracy, equality for women, and many causes that liberals support. The left’s fawning adoration of Ms. Sarsour is a vivid demonstration of the strange alliance between these two ideologically opposed groups. And this is far from an isolated phenomenon.

But the greatest threat we face comes from a third adversary: one that hides from view in our homes and churches.

—Adapted from the Introduction