Today, in the year 312, on the eve of the Battle of the Milvian Bridge, the Emperor Constantine saw a sign of the cross in the sky, and the words εν τούτῳ νίκα (in Latin, “in hoc signo vinces,” or, “in this sign you will conquer”).
Constantine won the battle, adopted Christianity as the religion of the Roman Empire, and the rest—as they say—is history.  You could say, then, that today is the birthday of Christianity as a global religion.
Above: “The Vision of the Cross,” by Raphael (or the school of Raphael), 1510.

Today, in the year 312, on the eve of the Battle of the Milvian Bridge, the Emperor Constantine saw a sign of the cross in the sky, and the words εν τούτῳ νίκα (in Latin, “in hoc signo vinces,” or, “in this sign you will conquer”).

Constantine won the battle, adopted Christianity as the religion of the Roman Empire, and the rest—as they say—is history.  You could say, then, that today is the birthday of Christianity as a global religion.

Above: “The Vision of the Cross,” by Raphael (or the school of Raphael), 1510.