So for a internship I am working through a Latin text from the 11th century Low Countries: De Diversitate Temporum ('On several things of this time') by Alpertus of Metz.
In it, Alpertus tells of a man in an inn who supposedly said the following:"...that the human soul amounts to nothing and that it leaves the body completely with the last breath to disappear into the air".
My Latin is pretty bad and the above sentence is my own translation of a Dutch translation of the Latin original. I will post the Latin at the bottom of this post.
Still, the writer condemns the man for saying this but it did make me wonder how common this denial of the continued existence of the human soul after the expiration of the body was during this time. I certainly had never heard of it in a medieval context.
The Latin:
"Audivi fidelem nostrum teferentem, quendam ex his miseris, cum in tabernis vino estuans derisisset sanctum hominem, ad hanc etiam miserabilem vocem erupisse, animam hominis nihil esse et in ultimo flatu in auras penitus evanescere". from: Alperti Mettensis, De diversitate temporum, ed. A. Hulshof (Amsterdam 1916), p. 24-25.
You might enjoy Episode 151 of the AskHistorians Podcast which features /u/Sunagainstgold interviewing Keagan Brewer about medieval atheism.
/u/mikedash has previously answered Was everyone religious in the old days, like Medieval Times, or were there irreligious people?
See below for more answers.
EDIT: All links now go to the intended places.
'Meeting a drunk man at an inn' is a rhetorical trope that is designed to facilitate and motivate the discussion of certain ideas, but not necessarily representative of actual beliefs. I could just in the same way discuss conspiracy theories by referring to a fictional person who beliefs that Elvis is still alive, albeit I doubt that there is a significant number of people who honestly reject the death of Elvis. (It could also be a tool to demonize people of certain ethnicities or religions - "look, they are all atheists and believe in absurd and immoral things!")
However, people believe all sorts of things on an individual and idiosyncratic basis, in the most extreme case when mental illness (or, in this case, drunkenness) is involved. The question is: can this belief be justified within a framework of rational and normative assumptions? Only then we can assume that it was sufficiently widespread.
As it was already pointed out, it has been argued that medieval people had few conceptual resources at hand to defend atheist belief. (Edit: there was late medieval / Renaissance skepticism that doubted the certainty of religious beliefs, see "The History of Scepticism from Savonarola to Bayle" by Richard Popkin, for example. But putting some skeptical pressure on religious claims is not the same as being an adherent of an articulated worldview or of metaphysical claims!) I want to elaborate this a bit. First, quite a few people in antiquity can be seen as atheists. These were mostly adherents to atomism (Leucippus, Democritus, Epicurus), which is important here. The atomistic worldview is in several important respects juxtaposed to the Christian worldview. Christians believe in a purposefully created and ordered world, and that living beings are distinguished from dead matter by virtue of being endowed by a soul - and ancient atomism rejects both claims. The world is a chaotic and contingent place and, if you take the position to its extremes (like it has been done in the 17th century by people like Descartes and De La Mettrie), living beings are just like machines that happen to think and feel by virtue of complicated organic processes.
Now it happens that in the early periods of medieval age, the documents of the ancient atomists were lost to even the scholarly circles. Only in the Renaissance age, some scholars were able to get their hands on Epicurus' writings again and ushered a new age of atheism that is closely tied to the resurgence of an atomist worldview. Stephen Greenblatt has written a highly readable account of this resurgence of atomism (The Swerve: How the World became Modern) and the corresponding resurgence of atheism.
So if a medieval atheist who could not have been an atomist were to reject the existence of the soul, he could not provide a coherent answer to questions like: "What's the difference between you as a rational animal and a dead body - or, furthermore, what's the difference between you and a pile of stones?" "The world is obviously an ordered place that has its own observable regularities. Where do you think these regularities come from, if they are not established by God?" Because only the atomist worldview, especially in its modern iteration, is able to provide answers to these questions that are not dependent on the belief in God and which adhere to the rationality inherent in the scientific worldview. Modern takes on how consciousness & feelings emerge from the functional organization of the body come up with early modern Epicureans like Descartes, Spinoza, and Hume; and ideas to justify natural laws and observable regularities in nature without referring to God come up at about the same time, late 17th century and 18th century with people like Spinoza and Kant. These ideas were simply not around in medieval times and could not be justified without stronger notions of mechanism, of perception, and of the objectivity criteria for universal judgments.
To indicate how rare actual atheists were, even during early modernity, consider the following anecdote. Its sources are not entirely clear and it might just be an urban myth, but I think it is very likely that at least something similar has happened. The English philosopher David Hume, who was not an outspoken atheist but most likely had profound sympathies for atheism, was visiting Paris in 1763-65. He frequently visited the salons of the French philosophes and discussed all kinds of matter with them. Baffled by the freedom and frankness with which his French contemporaries were discussing issues of religion, David Hume claimed that he had never met an actual atheist in his life! The Baron D'Holbach, one of the more radical proponents of the Enlightenment, exclaimed: "Why, monsieur, in this room alone there are seventeen of them!"
Edit: For an elaborate atheist treatise in medieval times, consider the anonymous treatise of the "Three Impostors": https://en.wikipedia.org/wiki/Treatise_of_the_Three_Impostors. This book derives its atheist claim from the idea that we have no independent criteria to evaluate the objectivity of any of the three Abrahamic religions and thus should be skeptical of all three altogether. As far as I remember, it does not deal with the two points I have sketched above and rather discusses the veracity and certainty of revelation.
Edit 2: One also could distinguish between two forms of atheism: one who rejects all kind of transcendence, and one who rejects the existence of God as a person, i.e. a being with personality and will. This distinction comes up again when people like Hobbes and Spinoza are later accused of being atheists, even thought they firmly commit themselves to the belief in a supranatural being as the source of all things. But this is the more abstract 'God of the philosophers', like the 'unmoved mover' of Aristotle or the Platonic Demiurge, which are not easily to be identified with the Christian God.