It is a fact that there are societal fads in terms of the place of religion in various cultures. In the 1920s and '30s, religion in the U.S., and in most Western countries, was seen as essentially irrelevant by most people. In the 1940, '50s and '60s, there was general respect and recognition of "moderate" religious views. This can be seen in movies and television; religious faith was a major part of the characters lives, for the most part. The '70s, '80s and early '90s saw a huge upswing (especially in the U.S.) of fundamentalist religious beliefs, and even cults of various kinds. From the '90s until the present, secularism and even atheism are everywhere, and religions have been put on the defensive in all Western societies.
In the 1930s, the question arose if it was possible to reconcile a strong Jewish identity with...atheism. As weird as this question sounds, the "cause' was taken up by a man named Mordecai Kaplan (1881-1983).
Kaplan began his life as an Orthodox Jew and rabbi. He was even a founder of the Young Israel movement; a network of Orthodox synagogues that exists to this day.He eventually came to JTS, the Conservative Seminary, where he became Professor of Homiletics (making sermons), a position he held for more than fifty years. He developed a very controversial theory. He saw G-d not as a being, but rather the sum of all existence; that which gives Man purpose and vision. (I am reminded of Yoda, in Star Wars, who tells Luke that the Force is the combined energy of all living things). Judaism is not a religion, but a civilization built around the idea of shared experiences, commitment and aspirations. He published two books in the 1930s based on this idea: "Judaism as a Civilization" and "Judaism Without the Supernatural". He was not, strictly speaking, an atheist. But he did not believe in the G-d of the Torah either. He preached for being a "Reconstructionist". That is, build a new Judaism where tradition has "a vote not a veto", but maintains the framework of traditional Jewish life as a backdrop to what later generations will build. Art, literature, all kinds of creativity were the cornerstones of Judaism for Kaplan. He was brutally critical of both Orthodoxy and Reform. The former for being too rigid, and holding beliefs which he thought made no sense in the modern world, the latter for betraying the foundations of our civilization. In JTS, he had many devoted and loving followers, but was bitterly opposed by much of the faculty for his abandonment of the Jewish concept of G-d. He kept strictly kosher. A colleague asked him why, since there was, in his opinion, no conscious G-d? He responded emotionally: "Because I'm a Jew. A Jew keeps kosher!".
He was denounced strongly by Orthodoxy as "the worst heretic of our times", with one rabbinic group actually issuing a formal ban of excommunication. A movement grew out of his ideas, which is known as the Reconstructionist Movement. It eventually formed its own seminary, and has synagogues in many locations around the world. Its membership is primarily an intellectual elite, and the movement is tiny compared with the others. It is characterized by acceptance of all expressions of Judaism, from Orthodox to atheist (it has been quipped by critics that they pray "to whom it may concern). Its seminary has been a haven for advocates of many forms of Jewish religious and cultural variations. Its influence goes far beyond its own ranks, as numerous Jewish "cultural" foundations and institutions have been inspired by it. Is it Judaism? I'll let you decide.