Sam Harris, on his podcast with Jordan B Peterson (episode 67) specifically defined what religion is to him around the 55 minute mark.
Now this is probably not his most precise definition as it is defined during a oral conversation with Jordan Peterson in an attempt to define what religion is in a more narrow pathway. But, to me, it seems like a good standard to see what is defined as a religion to Sam Harris.
Religion becomes religion for me when you begin to assert that certain things are true, specifically otherworldly things—the survival of the conscious mind after death, or the real existence of invisible agents which you can propitiate or fail to propitiate, and who have their eye on you, right, they care how you live, they care whether you masturbate, they care whether you pray to them or in what terms you do. And if you get any of that stuff wrong, you will suffer for it. You are in relationship to these things. There are some people who have relaxed their standards of commitment to revealed religion so much that they don’t really answer to that description, but most religious people most of the time, certainly most Christians and Muslims believe otherworldly and supernatural and superstitious things, which are, I would argue, in direct contradiction to reasonable things they might believe about how the cosmos works.