Opus Dei, if I remember the book correctly ( I haven't read it for a while ) was an extremely large group in the book. Spanning continents and so forth. The thing you have to consider though is that every religion has it's fanatics. It doesn't really matter what sect their in, what god or gods they follow. Someone is always going to take it to far.
What the "old man" was in, was the Illuminati. Which conspiracy theorists have been throwing around for years. No one really knows if they're still around. What Dan Brown was assuming is that they really weren't, the doctor scientist dude was just claiming that he was.
Do you have evidence of this? I'd love to read it I found the book rather interesting, both from a catholic point of view and a pagan one. Some ( i'm not going to say many or all ) of the facts in that book were true. I couldn't dispute or prove the Mona Lisa comment quasicartes made, but many of the references in the book are based on fact. For instance, Jesus was Jewish, and it is believed by some that what he was preaching was Jewish gospel, that modern day Christianity is based upon what the priests and church came up with later ( Satan, for instance, was made up by the church, and not Jesus ). So the theory that he was married isn't that far fetched. And considering that the Virgin Mary was named Mary, do you really think they'd name a prostitute after the mother of the son of God? o.O