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 based on his investigations. Luke pin-points and describes the beginning day.
¶“1I did the first account for you Theophilus, about all that Jesus began to do and teach 2until the day he was taken up. He had by the Holy Spirit given orders to the messengers he had chosen. 3And he showed himself alive to them by many proofs. He was seen by them for forty days after he had suffered. He spoke to them about the kingdom of God.” (Acts 1:1-3).
¶“44He said to them, 'These are the words I said to you while in your company. I told you that all things must come true that were written about me in the law of Moses, the prophets, and the Psalms.' 45Then he opened their minds to understand the scriptures.” (Luke 24:44-45).
¶“46He said to them, 'So it was written, and so it behoves the Christ to suffer and rise from the dead the third day. 47 And in his name, repentance and forgiveness of sins must be preached to all nations, beginning at Jerusalem.48 And you are witnesses of these events. 49Behold, I send the promise of my Father upon you. But wait in the city of Jerusalem until you are clothed with power from on high.' 9He led them out as far as Bethany where he raised his hands and blessed them” (Luke 24:46-50).
¶“4When they gathered, he ordered them not to leave Jerusalem, but to await the promise of the Father which, he said, 'You heard from me. 5John indeed immersed with water, but you will be immersed in the Holy Spirit not many days from now.'” (Acts 1:4-5).
¶“6Those who were gathered asked him this question: 'Lord, is it at this time that you restore the kingdom to Israel?' 7Jesus said to them, 'It is not yours to know the times or seasons which the Father has appointed in his own authority. 8But you will receive your authority when the Holy Spirit comes upon you, and you will be witnesses to me in Jerusalem, and in all Judea, then in Samaria, and to the ends of the earth.'” (Acts 1:6-9).
¶“9Following these sayings, he was lifted up while they watched, and a cloud took him from their eyes. 10While they looked intently at his departure into the sky, behold two men in white garments stood by them". 11And they said, 'Men from Galilee, why linger gazing skyward? This Jesus taken from you into heaven will come thus, in the way you have seen him go into heaven.” (Acts 1:9-11).
¶“51”And so it was, that while he blessed them, he was parted from them and taken up into heaven. 52And they worshiped him, and returned to Jerusalem with great joy. 53They were continually in the temple praising and blessing God” (Luke 24:51-53).
¶“1”When the day of Pentecost was in full swing, they were all together, fiercely united 2And a sound came unexpectedly out of Heaven, as borne of a violent wind, and it filled all the house where they were sitting. 3And they could see divided tongues like flames, and one sat upon each of them. 4They were all filled with the Holy Spirit, and began to speak with other tongues, as the Spirit gave utterance to them” (Acts 2:1-4).
¶“5Many Jews were staying in Jerusalem, pious men from every nation under heaven. 6The crowd gathered as the news spread, and they were confounded, because each heard them speak in his own language. 7And they were all amazed and marvelled, saying to each other, 'Look, aren't all those who speak Galilaeans?' 8How then do we each hear in the language of our birthplace? 9Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia, 10Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes 11Cretes and Arabians, we hear them speak in our own tongues the wonderful works of God!'” (Acts 2:5-11).
¶“12And they were all amazed, and were in doubt, saying one to one another, 'What can this mean?' 13Others mocked: they said, 'These men are full of sweet wine!' 14But Peter, standing among the eleven, raised his voice and declared to them, 'Men of Judea, and all you who dwell in Jerusalem, be this known to you and heed my words...'” (Acts 2:12-14).
Luke includes Peter’s sermon here (Acts 2:15-35).
¶“36'...Let all the house of Israel therefore know for certain, that God has made him both Lord and Christ —this Jesus whom you crucified.' 37Now when they heard Peter’s words, they were cut to the heart, and asked Peter and the other apostles, 'Brothers, what shall we do?' 38Peter answered them, 'Repent and be immersed every one of you. Do this in the name of Jesus Christ for the forgiveness of sins, and you will receive the gift of the Holy Spirit. 39This promise is made to you, to your children, and to all who are far away; yes, as many as the Lord our God will call.' 40With many other words Peter testified, and exhorted the people. He told them, 'Save yourselves from this twisted generation!'” (Acts 2:36-40).
¶“ 41Then those who were pleased to welcome Peter’s message were immersed. There were added that day about three thousand souls. 42They made a strong continuing commitment to the apostles’ teaching; and to fellowship; and to the breaking of bread; and to prayer. 43And on every soul there came fear; and through the apostles many wonders and signs occurred” (Acts 2:41-43).
¶“44All who believed kept company in all-inclusive fellowship. 45They sold things that they owned, and shared the proceeds out to everyone who was needy. 46Day by day they continued to be strongly united. They worshiped in the temple. In their homes they enjoyed breaking bread, sharing their meals with happy and simple hearts. 47They were praising God, and had approval in their whole community. The Lord kept adding to the congregation every day those who were being saved” (Acts 2:44-47).
At Jerusalem, on the 50th day following the Passover that had been observed just prior to the death, burial, resurrection, and ascension of Jesus.
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.
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.