How do we know that Jesus was born on 25th December, when historic criticism shows that we do not even know in which year he was born?
We do not know the exact date of Jesus birth. In the beginning there had not really been any interest in the date of his birth. People celebrated Easter and thus commemorated all of the mysterious events represented by the life of Jesus, especially for believing Christians. In the third century the wish to celebrate Jesus birth arose. It is the same phenomenon we can observe in the genesis of the gospels; first came the salvific works of the adult Jesus, the Messiah, only later the desire to trace events back to their origin and to look at what happened in the beginning.
Because the date of Jesus’ birth was not known it was possible to select the most suitable date. And spontaneously, the time of year when the days are getting longer was chosen. As long as can be remembered the 25th December and 6th January have been the dates for the first appearance of Jesus, and replaced Pagan festivals. But this is only of secondary importance. The deeper reason is much simpler and much more deeply rooted in human nature. With the coming of the light in nature we celebrate the new light that is never extinguished. It is a spiritual light. The Christian faith loves nature and happily connects with nature, but it is not a natural religion. The birth of Jesus is a historic fact; all of Christian history is counted accordingly. Year 1 is the year of Jesus birth, a wonderful vision through which in the sixth century monk Dionysius Exiguus the Little replaced the old calendar, which had been based on the foundation of Rome. Reading the evangelist Luke, according to whom Jesus was approximately 30 years old when he started his Ministry (Luke 3:23), Dionysius did not place sufficient emphasis on the word approximately, and as a consequence erred by four to seven years. But this is not of overarching importance. Even if Jesus was born a few years earlier, the deep meaning of the words Anno Domini, in the year of the Lord, remain: With Jesus begins a new time. (Slightly adapted from „Glaubensverkündigung für Erwachsene“. Deutsche Ausgabe des Holländischen Katechismus (Dutch original 1968).