The term Catholic Enlightenment refers to a heterogeneous phenomenon in Ancien Régime Europe and Latin America. It stands for the Church policy pursued by a Catholic enlightened monarch and/or his ministers as well as for a "reform movement" (which was a watchword for the adoption of Protestant assertions and principles of Enlightenment philosophers) within the Roman Catholic clergy to find answers to the ever-growing secularism of that period.
In contrast to the zeitgeist of rationalism, which in its pure form rejects revelation as contrary to reason, Catholic Enlightenment is characterised by the attempt of "reform-oriented" parts of the church to counter the onrush of mainstream Enlightenment. They endeavoured to reconcile what they saw as conflicting concepts of reason, which is seen as the sole source of truth by rationalists, and revelation as a disclosure of information by divine agency. It is by definition beyond the ordinary course of a rationalist conception of nature and was ipso facto a prime target for "enlightened" intellectuals and statesmen. In doing so, they challenged the very foundations of Christianity, and that consequently culminated in the complete suppression of Catholicism in favour of a Cult of Reason during the Reign of Terror in the French Revolution.
The secular Enlightenment was, however, by no means atheistic, but it declaimed certain tenets crucial to the Catholic Church as merely historic and man-made and therefore fictitious. Many of the most influential philosophers of that time, like the Encyclopédistes, Voltaire or Reimarus, were secularists or promoted a deist view: In a nutshell, they proclaimed that nature was the only revelation God has ever made and thus the preoccupation with any other alleged revelation was superfluous. Additionally the Bible (and the Old Testament in particular) was considered contradictory in itself to pure reason and to the perfection of God. Others, like Lessing, agreed with respect to biblicism and revelation, but he was more lenient toward "emotional (i.e. unenlightened) Christs" who were in need of the gospels to do good. Still, he detested what he perceived as ecclesiastical obscurantism and intolerance: He postulated a "Christianity of Reason" without the Tradition and dogmata of the Church, which he rejected. For Lessing it was all about the Education of Humankind and this very attitude was actually a stereotype of the "enlightened" elite: They saw themselves as guardians of reason and lowered Christ to principally a useful educator of virtue who was - in their opinion - just to be "freed" from what they saw as a fake and superstitious church-masquerade of the so-called "Dark Ages".