Atheism, in the broadest sense, is an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities. (1)
Based on the Ancient Landmarks that all regular masonic lodges agree upon, all members must profess a belief in deity. This means that atheists are not permitted to become members. However, this does not mean that one must belong to a specific religion or church. One can believe in a deity and not belong to a man-made church. As long as the person believes in deity, that is sufficient to become a mason.
Freemasonry is not a religion per se, but atheists cannot belong, said Brent Morris, a Masonic historian, editor of the Scottish Rite Journal, and a 33rd degree Freemason.
“This is an organization of believers,” he said. “When it was started on a formal basis in 1717, many historians believe that it was trying to bridge the gap between the religious civil wars that had been going on in England at the time. The Catholics would get in power and beat up on the Protestants; the Protestants would get in power and beat up on the Catholics; and everyone was beating up on the Jews.
“So when the Freemasons were formed, [they] said, ‘Here’s a group of men that agree that God is central in their lives, they can even agree that God compels them to do good in the community, then they can shut up after that.” That was a radical concept — that men could get together and agree on that fundamental level, and then get on with their lives.” (2)
(1) https://en.wikipedia.org/wiki/Atheism
(2) https://www.cbsnews.com/news/9-things-you-didnt-know-about-freemasonry/