1. Christianity is not a religion - Jesus often criticised the religious leaders and would have nothing to do with them.
2. Jesus was actually born in the autumn but the Roman church as it then was, decided to celebrate Him at the same time as the pagan saturnalia mid winter festival and it stuck.
3. To be a Christian you have to be "born again" as Jesus Himself said to a certain Jewish leader, then baptised in water and in the Holy Spirit (quite an experience I can tell you). As Jesus said to the disciples "you will then receive power".
4. Although all of this can be quite easily read in the Bible, the main cult churches do not teach it and so most people are completly clueless about "Christmas".