A thorough understanding of human history has been the hallmark of an educated man in the West since the Middle Ages. Herodotus is considered the first historian, but there is debate about whether the ancients had an appreciation for human history. Oswald Spengler argued that the ancients were ahistorical, as they lacked a historical consciousness. Perhaps that is true, but for our purposes, the point is that a classical education in the West has always included a thorough understanding of history.
Now, we live in a post-Christian age, but there remains a high degree of hostility to religion in general and Christianity in particular. As a result, religion is either left out of the history books or cast as some sort of malevolent influence. That’s a big problem as it is impossible to understand human history without understanding religion and its role in human affairs. This is especially true in the West where Christianity is arguably the most important feature of Western culture since the fall of Rome.
Obviously, knowing European history means knowing Christianity and that means knowing something about the Jews and Judaism. I’d recommend starting with Paul Johnson’s History of the Jews as it is very readable and covers the important bits without getting bogged down in academic posing. Given the outsized influence of the Jewish people on world history, as well as current events, you cannot be an educated man without fully appreciating the role of Judaism in world history.
Of course, a solid understanding of Christianity is important. You cannot understand the last 2,000 years of Western history without knowing Christianity and the history of the Church. There are so many books on the subject that you are spoiled for choice, but Paul Johnson’s History of Christianity is the one I’d recommend to every atheist that can read. The point is not to become a theologian. The point is to have an appreciation of and knowledge of the role Christianity has played in Western history.
Eventually, learning Christian history leads to how it evolved from its antecedents. Greek and Roman mythology is not just the starting ground for the fantasy role playing crowd. It laid the groundwork for the monotheism of the Jews to jump into the rest of human society. Once you fully appreciate what came to replace Greek and Roman theology, you can fully appreciate a classics like Edith Hamilton’s Timeless Tales of God’s and Heroes and Hilda Davidson’s Gods and Myths of Northern Europe.
Islam is a thornier topic as there has been a flood of books and articles on the Religion of Peace that are more about modern political topics than the history of Islam. A book worth reading to get a positive introduction to the history of Islam is Karen Armstrong’s Islam: A Short History. Like every book about a specific religion, it has its short-comings and critics, but it does the job. Another good option and maybe a companion to the other book is Reza Aslan’s No god but God: The Origins, Evolution, and Future of Islam.
The hard part about Islam for Western readers is we tend to look at religion through the lens of European Christianity. There was the world before Christianity and then the Christian era. Islam did not spring from nothing. It is the result of a long evolution of religion in the region. A good book to get a fuller understanding of that is Heirs to Forgotten Kingdoms: Journeys Into the Disappearing Religions of the Middle East. It provides a nice history of religion before Islam and a fuller picture of the complexity in the modern Middle East. It’s not all raging Imams and Jihadists.
As far as Eastern religions like Hinduism, Buddhism, Confucianism and Taoism, this is where it can get difficult for the Westerner. Thousands of books have been written on Eastern religions by hippy crackpots All of them are terrible and useless in terms of learning anything about these religions and their role in the development of those societies. The right answer is an old book by a long gone professor named Huston Smith. The book is The World’s Religions and it has good sections on these topics.
Belonging to a church or religious sect is not the same as joining the stamp club or volunteering down at the school. It is the defining feature of a person’s life and the defining feature of his group. For those who lack faith, this is not always easy to grasp. A good book on this subject is Dynamics of Faith by Paul Tillich. It’s easy to see why a great cathedral could be inspiring, but it is not easy to see what inspired men to build the cathedral. Understanding faith at the personal level is critical to understanding religion.
The point of learning about religion, as an intellectual exercise, is not to become a theologian. The point is to have a grounding in the basics in order to better understand human history. Belief is one of the oldest modern human traits. It is thought to have co-evolved with language. It is the main driver of human history and it remains a principle force in human affairs. There’s simply no way one can have an understanding of history, especially Western history, without understanding religion.