There are lots of theories and heaps of books written about the beginning of Christianity. A great many of these repudiate the account composed by Luke following his investigations. Luke pin-points the beginning day.
Where and when. At Jerusalem, on the 50th day after the Passover that was observed just before the death, burial, resurrection, and ascension of Jesus.
How and Why. Luke further shows that Christianity began on that day by the preaching of the gospel of Christ with power. Christianity was given the mission to preach that testimony of Jesus, beginning at Jerusalem, and then taking it into the uttermost parts of the earth. This was so the world could have forgiveness of sins.
Who. Luke also tells us who the first Christians were. They were about 3000 souls who received the gospel gladly, were convicted of their sins and convinced that the crucified Jesus was Christ. They repented of their sins and were baptized for forgiveness and to receive the gift of the Holy Spirit.
This is not the whole story of course. The life and work of Jesus beforehand, and the work of the apostles afterward must be considered. However in that setting, this one day is pin-pointed by Luke as the day that Christianity was launched as a new religion.
