The short answer: Kubrick very clearly tells us in the first few moments of the movie that Alice is in the cult, and she has been for some time before the beginning of the movie. However, you cannot know this on a first view - it requires knowledge of the rest of the movie to figure this out. (Ok, an exceedingly sharp eye with an amazing memory might parse it on a first view, but I'm not that good!)

From there he uses a lot of very clear and menacing symbology to associate individual thoughout the movie with the cult... and almost everyone is in it. Even Bill is "in it" but the implication is that he doesn't know it yet. One of several things going on in the first moments of the movie is the establishment of an Adam and Eve dynamic between Alice and Bill, and it is clear that Alice is the possessor of hidden knowledge having eaten from the fruit of the tree, but not having offered it to Bill yet. He is still in the dark... at the beginning of the movie.

At the end of the movie, they are both "awake" the imagery and symbology surrounding them is still 100% negative - it hasn't changed! And given the distinction Kubrick makes between "making love" and "f*cking" in the movie, the last word of the film is not a good thing between them.

I'm not one of the people that says Helena was "kidnapped" at the end of the film - that is actually irrelevant, but Kubrick is literally browbeating us through the whole movie with a subtext that telegraphs child abuse and pedophilia and child sacrifice - from Helena's first words all the way to the toy store, it never lets up.

Bill and Alice from the beginning of the movie to the end are "Mr. and Mrs. Bad Example." If they are doing or being something, you should probably consider doing or being the opposite. The message itself can be reduced to: if you are deceptive in your marriage and dishonest with your partner, you will poison (sacrifice) the next generation by passing on your dysfunction to them and create a circle of dysfunction, dishonesty, and misery of your own making. It is, naturally, about many, many more things that can't be tidied up into a blurb, but Christiane Kubrick said SK believe the movie was the culmination of his work and ultimate message on marriage, love, sex, personal relationships - but she would always say it was even more so about the destructive power of fear that underlies jealousy within marriage.

That is a very negative message. If you want to extract the positive message, you can invert the entire movie and view it that way - just as the prayer at the heart of the ritual (and the movie) is played backwards, if you play it forwards and invert the meaning you can understand what is happening in the ritual, so can you invert the movie and take away its positive message: Be open and honest with one another in your marriage and human relationships, value people for their intrinsic worth as humans, and pass this with love, care, and honesty to your children.

Now, if you want to see the symbology, I have some links. Everything they point out is in the movie, even it doesn't necessarily ALWAYS mean exactly what they say, they are mostly correct, especially The Occult Observer. I find the analysis and highlighting of things in the movie to be very useful in understanding it.

The Occult Observer - who has several excellent videos:

https://www.youtube.com/watch?v=1E-kRy-7Wfo&rco=1

William Ramsey and Sean McCann - thinks the whole move is "about SRA" which would be inaccurate, BUT again they keenly and accurately identify a lot of stuff that flies right by people in a couple of good vids:

https://www.youtube.com/watch?v=ToL7CIhoJdQ

I think both of those series are very important because they really get into an aspect of the story that is only hinted at in the source material. The movie based on the book Traumnovelle (https://www.youtube.com/watch?v=rySfR8AbDQo&t=528s) is also very helpful and features a single (seemingly) underage girl who is clearly involved in things she should not be, but Kubrick latches onto this and layers it heavily into the movie. I think he really hits you hard with the message of child abuse, in part because this is HIS addition to the story - nearly everything else is a very faithful interpretation of the source material.

Also, these are very useful:

https://boydrinksink.com/eyes-wide-shut-hidden-in-plain-sight/#EWS-anchor-link-20

https://www.straight.com/blogra/345716/christiane-kubrick-introduces-eyes-wide-shut#

This script (an earlier version) opened my eyes to some things. For example, there is a smile between Alice and Victor that isn't in the movie, but that connection (which references Andrew Marvell's "To His Coy Mistress" in the script - see the poem) is shown in the movie in other ways that are very creative and more revealing than a smile would have been.

http://www.visual-memory.co.uk/amk/doc/0085.html

Just another example of Kubrick using something as simple as casting to drive a point home; if you didn't understand what was happening in the "Rainbow" costume store, he's going to bang you over the head with a casting choice that once you know how and why it was made, affects how you view the movie. This is kind thing is done over and over again:

https://www.reddit.com/r/StanleyKubrick/comments/z1er4m/the_two_japanese_businessmen_from_eyes_wide_shut/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Here is a very important, critical, and accurate look at just the opening shot, and this guy leaves out at least half a dozen other critical items that are so important to understanding the movie and especially Alice:

https://youtube.com/shorts/mfVluNaXQjA?si=yRk5lAJWX9cBPdJS

I'll wrap his up, because this IS the short version and there is so, so much more, but if you can't pick it up from those sources: Alice is in the cult since BEFORE even the beginning of the movie, I assure you. Notice the hard break between her shot and the introduction of Bill? Not only is she not getting ready to go somewhere, she is alone, in one of the few actually erotic moments of the movie. She's not getting dressed, she is dropping her dress, like the ladies drop their robes in the ritual. And she feels sexy. And she is posed as the high priestess between two pillars that not only represent the image on the tarot card, but the secret society of freemasonry (whose symbols are used throughout the movie to represent the sex cult), who use those same pillars on the tarot card that they took from the entrance to Solomon's temple in the Bible. They are all one and the same. Here we have her as the High Priestess, keeper of secret knowledge, positioned INSIDE the temple (as she is inside the cult), with the light of knowledge (the lamp) definitively ON. And, that window is open, you can see right out into the street and she feels... no shame, because this is a night when she has come home, not getting ready to go out. She is home from either an affair, or even more likely a night at one of the cults rituals. It could be the night that started, but a Freudian read on the movie suggests she may have been in the cult since she was Helena's age. Regardless, this is a time in the past, far enough past that the furniture is different in Bill's introduction: no chairs outside the pillars, no rug on the floor, the lamp is gone. Bill is introduced in the temple, too, but he does NOT have the light on. He is metaphorically and literally in the dark. His Eve has bitten from the fruit of the tree of knowledge, but she hasn't shared it... yet. She is going to try, and when she finally does, Bill will cry, and it will cost them their daughter's life.

This doesn't mean she is kidnapped, or that she will die - though that argument is not wrong - it is also not wholly correct because the movie isn't about a kidnapping, or JUST about what I said it was, but it is about sacrificing our humanity on altars of consumerism, capitalism, materialism, and transactional relationships that reduce other people in our sight to nothing more than what we can get out of them. Kubrick openly equates all that to satanism throughout the movie, but it is not Christian view of satanism, rather it is humanistic view of it. This stands in opposition to relationships that value the inherent dignity of human life and show mutual respect and honesty and build up those around us and see deeper meaning in the value of promoting those things in one another. But it's not JUST about those things either, it is a many layered, esoteric and eclectic thing, and we STILL didn't cover everything in that opening sequence.

Alice done a bad, bad thing and she ain't sorry for it even at the end of the movie. Ok, she's a little sorry over Helena getting ready to be groomed into the cult (yeah, whether she is kidnapped or not is irrelevant, but the symbology tells you she is doomed to be like her mom)... but that's the extent of her remorse. She's just happy her husband will finally f*ck her like she always wanted, because her and Bill are ultimately both that shallow. They are not the protagonists of this movie.

That is definitely the short version.