In a sense, Christianity began long before Christ entered the scene. Old Testament prophets spoke of the coming of the Messiah, etc. However, while Christians claim these prophets are theirs, so do the Jews, and the Jews would be quick to say that Jesus was not the Messiah the Old Testament prophets spoke of. So it is difficult in a technical way on a Q&A website like this is to answer your question.
A satisfactory answer that I think most Christians and non-Christians would agree with is that its origins were in the person of Jesus Christ. His ministry inaugurated the Christian movement. No matter the denomination, I think it’s safe to say all Christians would agree that Jesus is the beginning of Christianity, not the apostles.
Personally, I hold to the Old Testament origin of Christianity. It is possible to claim that Jehovah of the Old Testament is Jesus Christ in the New. But historically speaking with no religious faith involved… Jesus’ first sermons mark the origin of Christianity, plain and simple.