In the readings you will often come across allusions to the contrast between revealed religion and natural religion (or deism). The distinction turns on what the nature of the evidence is for a particular religious outlook. Deism is a form of natural religion that was prevalent in eighteenth- and nineteenth-century Europe.
The evidence underpinning revealed religion typically consists of a god supposedly revealing himself (or herself or itself) to an individual or small number of individuals, perhaps on a unique occasion. The report in scripture of, for example, a burning bush speaking to Moses, where the voice is said to have had a divine source, is revelatory evidence.
Natural religion, by contrast, is based exclusively on non-revelatory evidence. In particular, it does not call for acceptance of the testimony of a single individual, an organisation or a religious text. It is ‘natural’ in the sense that the evidence for it is available to all of us as reasoning and experiencing human beings; it is not a special privilege of some subset of humanity.
A helpful way to think of the difference is to imagine what it would be like if all bibles, all priests, all mullahs, all torahs and all holy relics, etc. disappeared overnight, along with all our memories of their ever having existed. Any evidence of God's existence and character that would survive such a disappearance is natural evidence, not revelatory evidence. Natural religion consists solely of doctrines that are supposedly supported by natural evidence. You may be wondering what evidence for God's existence would remain once mosques, churches, popes, rabbis, and so forth are set to one side. As it happens there are several traditional arguments for the existence of God that do not appeal to the trappings of established religion (see below). It is to these that deists looked in defending their views.
For our purposes we can divide the main religious perspectives available at the time into four:
Atheists denied that there was any god.
Agnostics denied that there is sufficient evidence for or against God's existence; they abstained from believing either in his existence or in his non-existence. Hume insisted he was an agnostic rather than an atheist.
Deists believed that we have natural, non-revelatory evidence of God's existence and nature. Several of the philosophes had deist leanings, Voltaire, d'Alembert, and Rousseau being the most notable among them. None had anything but scorn for revelatory evidence.
Revealed religion was adopted by those who accepted the testimony of scripture, and in particular of the Bible as interpreted by the established churches.
The Enlightenment movement as a whole was an accelerated part of a drift away from appeals to authority that has continued in western culture to this day. Entrusting oneself and one's opinions to the dictates of an institutional religion was anathema to such thinking. All the old authorities, including the Church, were held to be subject to the authority of reason tempered by experience. Inevitably, there were exceptions such as Samuel Johnson, quoted above, but it is undeniable that pressure on the Church was growing in this period.