Christianity is the most popular religion in the world, with about 2 billion followers all over the world. Islam is the second most popular religion, with over a billion followers. Part of the reason Christianity is larger is because it is older, since it is about 2000 years old, as compared with Islam being only 1400 years old.
Another reason Christianity is more popular is because of its association with imperialism. First, it took over the Roman Empire. After the Roman Empire fell in 476 AD, the religion continued as the dominant ideology of the Byzantine Empire, which was a direct offshoot of the Roman one. Later, the Arabs built a vast empire using Islam as their unifying force, challenging the Byzantines. Finally, the Turks, another Islamic power, destroyed the Byzantine Empire.
Then the European powers spread their empires all over the world, taking Christianity with them. Islam remained relatively weak until two things happened to make it more powerful: European imperialism fell apart and oil was discovered in most parts of the Middle East. Suddenly, the Arabs became extremely rich due to their oil revenues, and with that wealth came the ability to spread Islam around the world. But in Europe, Christianity declined as the people became increasingly secular. The tragic events of World War II probably did more to destroy Europeans’ faith than anything else. Today, the USA is the most powerful Christian dominated nation in the world, but it is still secular in its government. And even here, religious influence is slowly declining.
I suspect that within another generation, Islam will surpass Christianity as the most popular world religion, but its power cannot last long, because oil is a nonrenewable resource. And when that oil runs out, the economies of the Middle Eastern states that depend on oil will break down, and so will Islam.
What can freethinkers, atheists, and secular humanists do to overcome this situation? They must do everything possible to end the dependence on oil, and indeed all other fossil fuels, and establish societies based on renewable and sustainable sources of energy such as wind, water, the sun and geothermal sources. Once at least some parts of the world are free from needing resources that are doomed to run out, we will have even less need for religions like Christianity and Islam.