It may be said of the eighteenth century that it was an age of faith as well as of reason, and of the thirteenth century that it was an age of reason as well as of faith.
Since eighteenth-century writers employed reason to discredit Christian dogma, a “rationalist” in common parlance came to mean an “unbeliever,” one who denied the truth of Christianity. In this sense Voltaire was a rationalist, St. Thomas a man of faith. But this use of the word is unfortunate, since it obscures the fact that reason may be employed to support faith as well as to destroy it.
There were, certainly, many differences between Voltaire and St. Thomas, but the two men had much in common for all that. What they had in common was the profound conviction that their beliefs could be reasonably demonstrated.
Carl L. Becker – The Heavenly City of the Eighteenth-Century Philosophers