I saw an interesting sight on my drive between two of my parishes the other day. Someone put up a fake hunter out in field and left it there for weeks. It looked something like a mannequin with a hunter's outfit; it even had the orange vest and a plastic rifle in its hands. Although I was not there to see it, I presume the man who did it showed up at a later time and shot his deer. I have never heard of doing this before, but it is a brilliant idea. Deer will run away when they see a person anywhere around them . . . unless they are used to him.
Get the deer used to the decoy that looks like you and you could possibly walk right up to them in that same area and they would barely take notice of you. It is bound to work with deer, and it is bound to work with us as well. No we are not going to be taken in by a mannequin that looks like Satan, but he does do the same kind of thing to us. Satan wants us to become numb to a large array of evils because he knows that if we recognize these things as evil then we will turn away from them (everyone turns away from what he believes is evil -- even if he is wrong about what is truly evil!).
If we get used to something in our lives, we will likely miss other things that are similar but much more dangerous. The trick is called desensitizing. It is what happens when we become dull to things. It is certainly the method used by the devil when he promotes the many and various evils that are well accepted today -- get people comfortable with small amounts, and eventually they will become comfortable with it becoming widespread. If we ourselves get used a certain thing (anything) long enough, we will take it for granted, and it can blind us to what is really going on.
It is not likely going to be the same for every person, but we are all subject to the temptation to ignore those things that we see repeatedly. Like those deer, we get to the point of saying "that thing has been around for a long time, it would never hurt me". That is precisely what the evil one likes to do to us. He wants to train us to let our guard down -- and leave it down permanently -- because we think that what he is doing will not hurt us.
Rarely does anyone choose adultery (for example) in a moment's notice. Instead, they are slowly made comfortable with small things, flirtations, lustful glances, affectionate touches, etc. Then, step by step, a person moves closer and closer to the actual commission of the grave sin itself. Once a person's resistance is broken down with the "lesser" things, he will eventually fall to the greater evil.
What things in your life have you become numbed to that are actually blinding you to the work of the devil sneaking up on you? Whatever it is, you most likely do not recognize it (that is the point of decoys!). It takes an extra effort to find these things, and that means that you either need to ask God to reveal it to us, or ask a spiritual advisor to do so (or, even better, ask both!), and (most importantly) be humble and willing to accept what you are told. Therefore, we all need to recognize that there is the potential that there is a "decoy" in our lives that we are convinced is harmless, but in reality it is merely setting us up for a great fall.
Are you watching? Have you asked yourself about your own presumptions? Only when we are more attentive to our circumstances will we be able to recognize those "decoys" that we have become accustomed to. Once again, this leads us to the need to do some healthy self-examination and then go to confession. Yes, those things are challenging and can even be painful, but it is not as though we were just playing some pointless sports game, we are dealing with our eternal souls.