The Church Times explains the tradition of chalking the doors this way:
The blessing and distribution of chalks with which houses are marked with a special logo commemorating the visit of the Magi is a Catholic custom that seems to have originated in and spread from Central Europe at the end of the Middle Ages.
On or near the feast of the Epiphany, and conveniently close to the begining of a New Year, the tradition has been to ask God’s blessing on homes and mark the door post or lintel with chalks that have been blessed for that purpose. The doors are chalked with the legendary names or initials of the three Magi, and the numerals of the New Year, connected with a series of crosses. The initials C, M, and B commemorate the Magi, Caspar, Melchior, and Balthasar, but also stand for the Latin prayer-request Christus Mansionem Benedicat: “May Christ bless this house.”
Marking our doors is a visible sign of our desire to make our homes places of welcome and sanctuary and of our faith that God is present with us, at home and wherever we go.
This Sunday, we will bless chalk to mark the door of our church and to distribute, along with a copy of the prayer, so you can mark your own doors. If you are unable to join us, use chalk blessed simply because it is part of God’s good creation and the prayer below:
Standing on your doorstep
The Lord be with you (or, if alone) In the name of the Holy Trinity:
And also with you
The wise ones followed God’s star to Bethlehem, seeking the saviour.
I will follow the star.
They found Jesus in Bethlehem and knew they had found the one they were seeking.
I will seek to recognize Jesus.
They fell down and worshipped him, offering him gifts of gold, frankincense, and myrrh.
I will offer our gifts to the Lord.
The door is marked 20+C+M+B+20 while you say:
Caspar, Melchior, and Balthazar followed the star of God’s son who became human two thousand and twenty years ago. May Christ bless this home and each person who lives or visits here.
Holy God, watch over my/our going out and my/our coming in throughout this year. Fill me/us with the light of Christ that I/we may grow in love, in wisdom, and in faith. Amen.