Rewatching "The Ninth Gate" and...
Wow. The foreshadowing on Balkan isn't subtle, and not in the way you'd think.
Here you have Boris Balkan, a publishing magnate as you'd only find in the late nineties, with what feels like a monopoly on the publication of books related to history, sociology and the etiological study of the occult. He's very much Mick Jagger's "Man of Wealth and Taste", minus the wit or charm - and he has his private collection set on a separate floor of his private multi-story penthouse. As you'd expect, he only deals in books related to or speaking of the Devil. Fair enough.
And you're telling me that this guy couldn't see past the obvious security risks of setting the passcode for his elevator and the passcode for his locked and likely temperature and humidity-controlled pressurized book-preserving chamber - to something as braindead-obvious as '666'.
If I'm Lucifer, this isn't the guy I want conversing with or representing me at the end of a massive and eye-opening wild goose chase involving old page-turners and eau-forte engravings. Right off the bat, if I have only Balkan and Corso as choices, I'm picking the heartless swindler who has enough sense to realize that attachments are dangerous in his business.
Y'see, armchair occultists have a bit of the ol' cryptophilia going on. They're interested in figuring stuff out - the why of it comes from various places. Balkan wants the Nine Gates for power, Liana Telfer thinks power's in what Aristide Torschia wrote in that book, the Baronness has honest intellectualism going for her but she's incredibly prideful about her attachment to her copy...
In the end, the only one who has a clear and ultimately disinterested motive for seeking the Ninth Gate is Corso - and that's after a path that led him from pointed self-interest to ravenous curiosity. Only Dean Corso ultimately has cryptophilia's purest expression.
Plus, I bet he's savvy enough that if you gave him Balkan's keypads, he'd have enough sense to pay a guy who actually knows a thing or two about security systems to design credentials that are randomly generated and that either cycle on a weekly basis or originate out of a new seed or hash for each iteration. If you've got a hard 3-symbol limit, like '666' and all 10 available characters in the numerical system, that gives you 720 possibilities.
Out of 720 possibilities, Boris Fucking Balkan went with '666'.
He deserves to burn. That's an amateur move if there ever was one. Besides, even if you really had to go for something evocative, you could pick at several other instances of demonic numerology that are a lot less mainstream, like '333' (the inversion of Christ's purported time of death, usually recorded around 15:30 in Military Time) or pick at "evil" numbers in the way the term is employed in number theory. In that sense, "evil numbers" are numbers that expand to even numbers of 1s, when rendered out in binary.
So, yes I've seen the movie a few times and read the book, but in retrospect, Balkan being positioned to more or less hoist himself by his own petard is fairly obvious.