An important question we must ask is how do we recognise Satan and his forces? The Bible tells us that we should be sober and vigilant, because he wanders around like a roaring lion, seeking to devour us. This is no small threat. The devil is not some annoying mosquito we can just swat away at our leisure. Rather, he is a horrible horrible monster. A vicious, cold blooded predator.
And he doesn't look like it. Just as regular creatures in nature, camouflage is one of Satan's best tools. He does not look scary, prompting us to be as sober and vigilant as the Bible warns us to be. Rather, he looks harmless. Helpful, even. He looks like the kind of being we can lower our guard around. Like we can actually trust him.
As it turns out, there are some quite plausible theories about the origins of certain religions regarding Satan's direct influence. The false prophet Muhammad claimed to be receiving his revelations from an angel who appeared to him in a cave. The false prophet Joseph Smith likewise claimed to have been visited by the angel Moroni. A few holes could be poked in the theory that Satan was both of these angels, but when we read 2 Corinthians 11:14-15, we see that it is at least within his capabilities.
In the end, it does not matter who these angels were. Whether Satan in disguise, or just fictional characters, these two religions are demonstrably false. We can tell this by studying the very true word of the Lord Himself. This is how we stay vigilant and sober. Constantly study the word, and compare everything we hear to it. When the sayings of the world do not match up with the word of God, they are lies from the pit of Hell, and must be discarded, lest they drag many souls to the pit with the demons who invented them.