"The devil with one-third part of the spirits of our Father's Kingdom got here before us, and we tarried there with our friends, until the time came for us to come to the earth and take tabernacles [physical bodies]; but those spirits that revolted were forbidden ever to have tabernacles of their own. You can now comprehend how it is that they are always trying to get possession of the bodies of human beings; we read of a man's being possessed of a legion, and Mary Magdalene had seven.
"You may now see people with legions of evil spirits in and around them; there are men who walk our streets that have more than a hundred devils in them and round about them, prompting them to all manner of evil, ... and if you were to take the devils out of them and from about them, you would leave them dead corpses; for I believe there would be nothing left of them.
"... You can see the acts of these evil spirits in every place, the whole country is full of them, the whole earth is alive with them, and they are continually trying to get into the tabernacles of the human family, and are always on hand to prompt us to depart from the strict line of our duty."
-Brigham Young
I should always both pray and be obedient to God's commandments if I want to make sure the spirits that are influencing me are clean.