I answered essentially the same question two months ago: here

We know that there were some. There was a non-religious school of Islamic Scholarship. While no original documents survived we know from preserved critiques and debate that Ibn al Rawandi argued for the primacy of intellect and asserted that spiritual revelation wasn't knowledge. There were several other who followed similar lines of reason such as al-Warraq, Al-Razi, and Al-Ma'arri. Now, it's important to note that as far as we can tell they weren't arguing against the existence of God, so they are probably best defined as Agnostics.

In Europe we don't have much in the way of a first hand account of atheism, muddying the issue was that it was one of those trumped up charges that people brought against political rivals along side devil worship and witchcraft. This means that people were familiar with concept, at the very least. It was even leveled against Pope Boniface VIII, at least according to Draper's History of the Intellectual Development of Europe. I'm not fond of Draper, after all he did write History of the Conflict between Religion and Science and popularized that sentiment despite the thesis not being supported by evidence. Draper wasn't an atheist, he just really wanted to call Catholics dumb in the most academic ways possible.

An intriguing possibility was a bit in Jacob Grimm's Teutonic Mythology that suggested that Vikings sometimes turned away from religious expression and were called "godless". We have a primary source of this in the Icelandic Hrafnkel's saga in which the titular character ceases sacrificing to the gods after his Temple to Freyr is destroyed by a raid.

We don't really have much of any direct sources from the peasantry of any of these things, but apparently it wasn't overt or common enough to work its way into the cultural or war propaganda of the period. It's far more likely that any atheists kept it largely to themselves and went entirely unnoticed by history.