What event in AD 312 allowed Christianity to spread across Europe?

Constantine's conversion to Christianity in AD 312 was a pivotal moment in the history of the religion, significantly contributing to its spread across Europe. Prior to this event, Christians faced persecution and the faith was often practiced in secrecy. Constantine's public endorsement of Christianity, marked by his conversion, led to the Edict of Milan in AD 313, which legalized Christianity and allowed it to be practiced openly within the Roman Empire.

This conversion also established Christianity as not only a tolerated religion but one that had the potential for political support and patronage. With the backing of the emperor, resources were allocated towards the construction of churches, and the religion began to attract a wider following, including people of influence. This ultimately set the stage for Christianity to become the dominant religion in Europe as the Roman Empire expanded and eventually transitioned into the Holy Roman Empire, further influencing culture, governance, and society across the continent.

The other events mentioned, such as the fall of the Roman Empire, the invention of printing in China, and the signing of the Magna Carta, while significant in their own contexts, did not directly lead to the initial spread of Christianity across Europe in the same manner or timeframe as Constantine's conversion did.