I think Voltaire may well have been anti catholic as opposed to atheist, but I will leave that for you to decide for yourself
Si Dieu n'existait pas, il faudrait l'inventer
If God did not exist, it would be necessary to invent him
This statement by Voltaire was so famous that Flaubert included it in his Dictionnaire des idées reçues, and it is still frequently quoted today. But where does it first appear? And what precisely did Voltaire mean when he wrote it? Does it imply, as most of Voltaire's parrots seems to suppose, that God is a fictitious being, created for the comfort of the human race?
In its original form, the statement first appeared in a verse epistle from 1768, addressed to the anonymous author of a controversial work, The Three Impostors. According to Voltaire, this was a virulently atheistic text that denied the existence of a divine being. He found that this was an extremely dangerous work since it put into question a notion that was very useful for society: the idea that criminals would be punished in the afterlife. Thus, even when policing was insufficient, there was a strong deterrent against crime.
In this poem, Voltaire develops in a general way the idea that the existence of God (or the belief therein) helps establish social order. He then goes on to boast of his own role in eliminating prejudice and injustice in the eighteenth century. In its final sections, the poem turns to personal satire, as Voltaire attacks some of his favorite enemies.
As such, the text shows many of Voltaire's complexities, both in his philosophy and in his personality. Although he attacked the abuses of the Catholic Church throughout his life, he also spoke as a defender of religion
Voltaire, Epître sur Les Trois Imposteurs