I remember reading in Evidence for Christianity that Voltaire scoffed at Isaiah 53 being written before Jesus’ time, and he claimed he would believe if someone could prove Isaiah 53 was written beforehand. I find that fascinating, what is the title of Voltaire’s work containing that quote?
I did a fair amount of looking around. My research found a couple of people referring to this Voltaire statement, but they did not reference a location in Voltaire’s writings. It sems like people are quoting other people, and not looking for the primary source. I did find this reference. “It has been related that Voltaire, the great French infidel, said if he could be convinced that the fifty-third chapter of Isaiah is genuine, he would concede that at least one prediction of the prophets was fulfilled” (McGarvey, Sermons, p. 128).
I do not have access to McGarvey’s sermons. He was a Church of Christ preacher and author in the late 19th century. I somewhat doubt that if you found this sermon it would have the reference anyway, because it is a sermon, not a scholarly treatise.
I think that, unless you can find a good source for this claim, you might want to be cautious about using it. The fact that I could not find evidence for it in 30 minutes of looking around does not prove much, and you might want to do some more looking around, but I would be cautious about using this claim in the mean time.