"The Truth went a stage further, holding that this was difference that could be made to make a difference. What was necessary was for people truly to believe in their hearts, in their souls, in their minds, that they really were in a vast simulation. They had to reflect upon this, to keep it at the forefront of their thoughts at all times and they had to gather together on occasion, with all due ceremony and solemnity, to express this belief. And they must evangelise, they must convert everybody they possibly could to this view, because - and this was the whole point - once a sufficient proportion of people within the simulation came to acknowledge that it was a simulation, the value of the simulation to those who had set it up would disappear and the whole thing would collapse.
If they were all part of some vast experiment, then the fact that those on whom the experiment was being conducted had guessed the truth would mean that its value would be lost. If they were some plaything, then again, that they had guessed this meant they ought to be acknowledged, even - perhaps - rewarded. If they were being tested in some way, then this was the test being passed, this was a positive result, again possibly deserving a reward. If they had been undergoing punishment for some transgression in the greater world, then this ought to constitute cause for rehabilitation.
It was not possible to know what proportion of the simulated population would be required to bring things to a halt (it might be fifty percent, it might be rather smaller or greater), but as long as the numbers of the enlightened kept increasing, the universe would be constantly coming closer to the epiphany, and the revelation could come at any point.
The Truth claimed with some degree of justification to be the ultimate religion, the final faith, the last of all churches...
...It could also claim a degree of universality that the others could not. All other major religions were either specific to their originating species, could be traced back to a single species - often a single subset of that species - or were consciously developed amalgams, syntheses, of a group of sufficiently similar religions of disparate origin..."
The "Truth" is the prevalent religion in the galaxy. Of course, any description of this religion that may seem similar to Christianity is purely coincidental (right). What I like about this book specifically and the sci-fi/fantasy genre, in general, is that you may seem like you are talking about one thing, but you are really talking about something else altogether. The cloak of science fiction gives one license to explore controversial subjects stealth-like. We've certainly seen that in things like His Dark Materials (The Church, free will) by Phillip Pullman and in Battlestar Galactica (fundamentalism, war on terror, torture, etc.).
The above passage and the following one also show that Banks is well-versed in philosophy. In this case, Bostrum's simulation thesis.
" ... Any theory which causes solipsism to seem just a likely an explanation for the phenomena it seeks to describe ought to be held in the utmost suspicion."
The Algebraist is also just a darn entertaining book that doesn't assume that you take any more out of it than that. But if you want the added meaning, you don't have to dig very far. You will not find much better fiction that explores the importance of rationality so well.