“The discipline contained in daily obedience and clean living and wholesome lives builds an armor around you of protection and safety from the temptations that beset you as you proceed through mortality.”
Elder L. Tom Perry, “Called To Serve”. Ensign, May 1991
I think the keyword here is discipline. If we don’t remain diligent and vigilant in doing what we must do in every aspect, then satan gets his puny foot in the door and is able to keep us from doing the things we must. Even if we slacken in one area, that’s enough for him to weaken us in every area.
This digression doesn’t happen all at once but little by little. The same way we learn and grow, gain knowledge, build faith, namely little by little, is the same way we end up going the other way. If we aren’t moving forward, then we are moving backwards. There is no standing still. We may think “oh I’ll just relax and rest for a bit”. But once we relax and let our guard down, satan gets in, however small the crack in our armor. That is an invitation to him. Then, he goes to work. If we aren’t adding to our armor, we are taking away from it.
I’m contrast, when we remain vigilant in living in accordance with how our Heavenly Father wants, then our armor gets stronger and is added upon, little by little, so that it ends up covering more of us until our suit of armor is complete. That suit if armor can take a lifetime to complete. The less we are exposed, the less satan can get in, of course.