It was a test to see if he knew their rules.

What an exercise in futility that would be. You need a microscope just to see Tom Cruise himself, and we definitely don't have the technology to be able to cause enough magnification to see his tiny wang.

Because it was penis inspection day.
VLAD LOVES UNDERAGE BOYS JUST LIKE HIS DADDY USED TO.

The suggestion is that someone involved in the ritual party placed the mask there, either by themselves or by hiring someone else, as they did to have Bill followed for example. Security cameras or guards or locks all mean nothing, it is to show again that these people, who ever they are, are very very high up and powerful, yes they absolutely could break into an apartment building unnoticed to place a mask as a threat. Also, I think it was quite apparent that they were distressed by that fact, we see Bill break into tears and sobbing, the next morning while we skipped his confession we can observe Alice is very upset…not just by Bill's "adventure" but by the real threats of consequences. Going back to why they told him to remove his clothes, there are a couple reasons…obviously the first is to make him vulnerable and shame him (interesting to note that his reality is mirrored within Alice's dream at almost the same time), this likely being only the first step, as someone else said rape may very well have been the next, but this isn't simply punishment, just as the rituals were not simply a party, not simply an orgy, there is more to it. They may have been initiating him. This does not mean he would be in equal standing with the elite members, but he may have become an underling, just as the women in the ritual were, permitted to be there to be used and abused, this may have become his fate…The answer of whether he would have been killed or not we will never know but we do know WHY he wasn't within the film's lore, and that is because during the ritual party he was "redeemed" by the mysterious woman and "when a promise is made here there is no turning back". The rituals being performed in this party are not just frat party shenanigans, it is serious and very dark magic. People familiar with practices like this will know, it goes far beyond the face value debauchery. So, as the rituals are serious, so too are the rules, and by those rules he was saved from whatever fate would have befell him BUT that certainly doesn't mean he'd be allowed to cause trouble for them or try to expose them, hence the very menacing threats. The mask on the bed is a demonstration, "yes, we can indeed place the most dire of consequences on you and your family" another promise made during the ritual.
Cast In The Name Of God…Ye Not Guilty

No I don't take any drugs , not sure what that has to do with anything, there was no need to be rude, I have not been rude to you at, just stating my view.
Btw the character name was Alice not Nicole. I didn't say that the actress Nicole Kidman was involved! lol
There is no proof that Alice was involved but there is no proof that she wasn't either….there are many parts of the movie that hint that she was involved and many dark theories to this film . There are many people that believe this theory do some research online.
Like I said in my other posts , Bill's wife Alice is clearly hiding something… how did that prostitute Domino know what Bill's last name was when he never mentioned his name to her once? Watch that scene again, when Alice call's him to check up on him, Domino asks "Is that Mrs. Harford"? He never told her his name…..ad why was Alice's eyes open when she was in bed when Bill came home from the party just before she had that dream with all the guys "Having sex with her"
Also, as I said before, why did they not seam scared all that someone just broke into their house to put the mask there if that was the case? I don't think I would be calm worrying about going Christmas shopping if my house just gotten broken into!
You don't have to agree with me, but remember Stanley Kubrick always wrote things to get his audience to think with different twist and turns, he was known for that.

You must take some heavy drugs.
You say that they couldnt have gotten through security but she couldve?
Nicole wasnt in no way involved. I dont know how you get the stories crossed.
Yea,
Okay.
Also,
That was the Point. That they can bypass anything and they rule everything. So theyve couldve easily killed him but they didnt. They left the mask as a Warning.
Without strife, your victory has no meaning.
Without strife, you do not advance.
Without strife, there is only stagnation.

I really don't think it was the cult that put the mask on his bed , there is no way they would have been able to get in his condo with security and all… plus I'm sure when Bill and his wife Alice discussed the mask it would have came up that she never put the mask there if that was the case. They didn't seam in any way scared that anyone at broke into their place at the end! Alice obviously put the mask on the pillow- whether or not she was part of the cult we still don't know for sure (Though I'm pretty sure she was) but I think it's safe to assume she was the one who put the mask there either way.

The Mask.
Without strife, your victory has no meaning.
Without strife, you do not advance.
Without strife, there is only stagnation.

It doesn't make it clear they are gonna kill him at the end , what do you mean?

Um,
No.
They were going to Kill Him,
And the ending Makes it Clear.
Without strife, your victory has no meaning.
Without strife, you do not advance.
Without strife, there is only stagnation.

That's what they wanted us to think was gonna happen in that scene, and it seams that women saved Bill's life by sacrificing herself. However, it turns out that was all staged just to scare him once they find out who he is… as his friend tell him at the end it was set up just to make sure he never tells anyone what he saw that night.

The answer is too horrible to contemplate.
They wanted him vulnerable and terrified. The demons they worship eat terror. Then after he was raped and abused, he would have been killed.

They wanted to identify him before proceeding with the tickling.

Gang rape.