What about a Jewish European, a Buddhist European or a Muslim European? Should they have to choose Aquinas too? I don't say about Voltaire and d'Holbach, but a Constitution is supposed to be valid for all European citizens, and talking about any roots, religious or not, would be making it about ten times longer than it is and still lead to people being left out.
I can't say I know d'Holbach at all, but can't his atheism and determinism be cancelled out by the fact that Voltaire was neither? And that, if d'Holbach was not a supporter of absolute monarchy, can it cancel out the fact that Voltaire had to eat and save his neck, too?
The Enlightenment is not only overrated, it is the source of some of the worst intellectual crimes of succeeding ages.
Christianity, on the other hand, led to perfect intellectual peace, especially when the Inquisition kicked in. I can totally see your point.