A brief google search, using the terms +"reign of terror" +atheist, brought up this link:
http://www.geocities.com/robespierre_et_saintjust/history.html

The excerpt in question:

"The issue of religion and national spirituality was also one heavily debated. In November of 1793, the Convention aspired to rid France of Christianity, which it believed was an unsound foundation. They proclaimed a new Religion of Reason, replacing the Roman calendar with their Revolutionary calendar, and closing down churches. Jacques-Ren? H?bert, an atheist, and his political club backed this decision, although the majority of the common French people heartily disagreed with it. Robespierre, realizing this radical movement could be the undoing of the entire Convention, fought against it. March of 1794 brought about the arrest and execution of H?bert and his followers..."

[Incidentally, your post on this website was the 41st link on that google search.]

In any event, while some radical atheists did have a hand in part of the Reign of Terror, they can hardly be blamed for it.

When fascism comes to America, it will be wrapped in the flag and carrying the cross."
--S. Lewis