Voltaire advocated deism and vehemently opposed atheism. As far as I know, one of the most important reasons he gave was pragmatic, that the uneducated lower class must be kept believing in god, otherwise ethics and social order would collapse. Hence his famous quote that if god would not exist, we would have to invent him.
I am confused if he meant that the lower classes should be taught deism instead of traditional Christianity, or if deism would have be kept hidden from them. In the latter case, his pragmatic argument for deism would be pointless.
But otherwise, could one really assume that teaching deism is an important contribution to teaching ethical behaviour? The usual argument is that belief in god contributes to ethics since believers expect that god will punish evildoers after death. Traditional Christianity claims to know very precise rules how that judgement will happen.
So let's imagine that a deistic teacher describes a god who created the universe like a clockmaker and does not interfere after that. The teacher could still claim that this god also punishes evil in the next life. But what if a pupil ask how exactly we know that? Is a clockworker god allowed to send revelations? If the teacher argues that we can conclude god's judgement of good and evil according to the rational structure of the universe, the pupil could point out that even then we cannot know if the deistic god's punishment will be harsh or benevolent; that kind of god might be extremely forgiving and quickly resocialise all sinners into heaven. So I would conclude that deism can hardly be expected to provide much deterrence.
Has Voltaire ever discussed questions like that?
I guess one could view the matter more in the light of a historic political process, that effectively deism in that era hoped to masquerade as a mild and moderate form of Christianity, and the Christian establishment to a certain extent went along with that, offering some sort of tacit deal like "if you deists attack atheism just as vehemently as we do, we will give you a seat in the supervisory board as the token sceptic". But that would mean Voltaire's reasons were purely political. My question is, did he (or for that matter, other deists discussing the same topic) give solid philosophical reasons for preferring deism to atheism in regard to ethics?