|An image (Ѻ), from a 2015 Christian blogger, depicting a believer as one who has god mixed into their understanding of atoms and atomic theory, and the unbeliever as one who does not have god beliefs, who uses the diagram to question the believer ideology that “a deep friendship with an unbeliever will pull the believer down”, spiritually speaking.|
In 1772, Joseph Priestley published Institutes of Natural and Revealed Religion, wherein he argued against atheism.
In 1782, a “Mr. Hammon”, later said to be English chemist-physician Matthew Turner (Ѻ), or possibly two authors combined, published Answer to Dr. Priestley’s Letters to a Philosophical Unbeliever (Ѻ) , in which, by stating the following, becomes the first overt or avowed atheist in Britain: 
“As to the question whether there is such an existent being as an atheist, to put that out of all manner of doubt, I do declare upon my honor that I am one. Be it therefore for the future remembered, that in London, in the kingdom of England, in the year of our Lord one-thousand seven-hundred and eighty-one, a man has publicly declared himself an atheist. ”
Here, firstly, we can compare this “public declaration” of atheism to the famous 1763 Hume-d’Holbach dinner party encounter, wherein in David Hume, upon first encountering Baron d’Holbach, says that he does not believe in atheism “because he had never seen one”.
Here, secondly, we see the term “philosophical unbeliever” equated to and or synonymous with “atheism”.
In c.1835, German poet Heinrich Heine was using the term “Unglaubensgenossen” (unbelievers) and or “Unglaubensgenoss” (unbeliever) in reference to Benedict Spinoza.  Jennifer Hecht (2003), of note, incorrectly asserts that Heine coined the term “unbeliever”, which is not the case, as the Mr. Hammon usage (above) shows.
In 1927, Sigmund Freud, in his The Future of an Illusion, was citing the following statement by Heine, whom he refers to as a “fellow unbeliever”, as something he can say without regret:
“We leave heaven to the angels and the sparrows.”— Heinrich Heine (c.1850), Publication
In the 21st century, the query "are you a believer" is a non-offensive way of asking do you believe in god.
1. (a) Hammon. (1782). Answer to Priestley’s Letters to a Philosophical Unbeliever (“I do declare”, pg. xvii). Publisher.
(b) Berman, David. (2013). A History of Atheism in Britain: From Hobbes to Russell (pg. 110). Routledge.
2. Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 451). HarperOne.
3. (a) Freud, Sigmund. (1927). The Future of an Illusion (editor: Peter Gay) (pg. 62-63). Norton, 1961.
(b) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (Hume-d’Holbach dinner party, pg. 352; unbeliever, pg. 451). HarperOne.