Is Matthew 1 the start of Christianity?
No. Entering Matthew chapter 1 is not yet Christianity; it remains an extension of God dealing with the nation of Israel according to Old Testament covenants and promises. It is all Jewish, under the Law, with the Temple still operating, and early believers did not shed their Judaism.
Les explains that the average church person assumes that as soon as you get into Matthew chapter 1, this is Christianity, but that is not the truth. At that point it is not yet Christianity; it is still an extension of God dealing with the nation of Israel based on all those Old Testament covenants and promises. Christ will come on the scene, as announced in Luke chapter 1, not first with the message that He is going to the cross to die for the sins of the world, though that is in the mind of God, but first and foremost to fulfill the promises made to Israel. So it is all Jewish, under the Law, and the Temple is still operating. Even those who become believers and followers of Christ do not shed their Judaism; they keep up Temple worship and maintain the Law, because no one says they are not under Law until somewhere down the road. Les also emphasizes that the Bible is a progressive revelation.