Whats the most popular religion in the world?

“Paul then stood up in the meeting of the Areopagus and said: “Men of Athens! I see that in every way you are very religious” (Acts 17.22).

In terms of numbers, Christianity is the world's most popular religion, with approximately 2.1 billion followers. This includes just over a billion Catholics, about 350 million Protestants, and the Orthodox and Anglican faiths. The religion statistics resource Adherents.com hosts a fascinating pie graph that breaks down the world's major religions by population.

Islam is the world's second-largest religion, with roughly 1.3 billion followers -- including 940 million Sunnis and 170 million Shiites, as well as those belonging to other Muslim faiths like Sufism. Depending on your resources, the third-largest group is either Hindus or atheists. Both hover just under the billion mark. Traditional Chinese religion (which is related to, but distinct from, Buddhism) takes fourth place with 400 million, and Buddhism trails close behind.

The Hindu organization Om Sakthi hosts a nifty timeline tracking the origin of the world's religions. Islam's rapid growth is impressive, especially considering it's one of the younger major faiths (it was founded in A.D. 622). Right now it's considered the fastest-growing religion in the world.

FYI: I hesitate to use the word ‘religion’ either of the content of the Christian faith or of its expression in worship and service. Mainly, because it is my conviction that Christianity is not simply one among many religions. It differs qualitatively from all others in that its content is divinely revealed and its outward expression by believers is not an attempt to secure salvation but a thank-offering for it.

The word religion is ambiguous. Its etymology is doubtful. Cicero refers it to relegere, to go over again, to consider. "Religio" is then consideration, devout observance, especially of what pertains to the worship and service of God. "Religens" is devout, conscientious. "Religiosus," in a good sense, is the same as our word religious; in a bad sense, it means scrupulous, superstitious. According to Augustine religio is the ground of obligation. It is that which binds us to God. Subjectively, it is the inward necessity of union with God. Commonly the word religion is used in its objective sense as when we speak of the Pagan, the Hindu, the Buddhist, the Mohammedan, or the Christian religion. Subjectively, it expresses a state of mind. What that state characteristically is, is very variously stated. Most simply it is said to be the state of mind induced by faith in God, and a due sense of our relation to him. As more concisely expressed by Bretschneider, "Faith in the reality of God, with a state of mind and mode of life in accordance with that faith." Or, more vaguely, "Recognition of the mutual relation between God and the world" (Fischer), or, "The recognition of a superhuman causality in the human soul and life" (Theile). "Faith founded on feeling in the reality of the ideal" (Jacobi). "The feeling of absolute dependence" (Schleiermacher). "The observance of the moral law as a divine institution" (Kant). "Faith in the moral order of the universe" (Fichte). "The union of the finite with the infinite or God’s coming to self-consciousness in the world" (Schelling).

The above definition of the word religion is drawn from Hodge, C. (1997). Systematic theology. Originally published 1872.)

“If anyone considers himself religious and yet does not keep a tight rein on his tongue, he deceives himself and his religion is worthless. Religion that God our Father accepts as pure and faultless is this: to look after orphans and widows in their distress and to keep oneself from being polluted by the world” (James 126-27).