It is a very difficult question to answer in just a few words because the history of Christianity is very complicated and vast. The first church ever was probably built sometime before the year 70 AD in Jerusalem, but it was demolished by the Roman Empire shortly after it was built. The next Christian church was built in Palestine and from there on, it spread around the globe through trade and through the preaching of the priests and disciples who traveled to different places around the globe. It is not quite feasible to mention an exact date or year from which Christianity started because there is no such document present today, that can confirm it. Of course, the term “Christianity” stemmed from Lord Jesus Christ and therefore it is concluded that the religion started during the first century. But whether Christianity as we know it, began while he was still alive or if it began immediately after his crucifixion, is a debatable matter. “Christianity” is just a term and although this term may have been adopted later, Christianity began when Jesus walked the earth with his disciples. Christianity had a hard time during its initial years under the Romans who had banned it, until of course Constantine came into power in the 4th century. It was again Constantine who made it the official religion in the year 380.
“Christ” is a Greek term which means “the anointed one”, but it was in all probability, given to the followers of Jesus as a sarcastic insult, by the non-Christians in Antioch. The name was not taken in a negative way, but was accepted instead and without protest as they believed in what Jesus had taught them. However, “Christian” was just another term that the followers accepted alongside “Brothers”, “Believers”, “Saints” or “followers of the way”, but the term did not have any impact on what they believed or followed. By the year 63 AD, “Christianity” was the most popular term that began to be used to describe the followers of Lord Christ. Perhaps the most adequate answer to the question in concern is that Christianity started the day Jesus had his first follower because Christianity is only a term used later to represent all that he taught, in one word.