In January of this year, I did this for the first time (OK, the chalk would not write clearly on the paint of the door frame, so I compromised!). This was partly prompted by my frequenting a parish served by priests from Poland and partly by my having neighbours from other European countries. I did think about how I would explain the inscription to anyone who asked me about it, and it is interesting to see where I got to.
I ended up somewhat dissatisfied with explanations such as that about blessing of the house, though the custom does clearly have this aspect to it. They seemed a little bit fanciful, and I felt they were trying to "read into" the practice something that might not have been there in its original inspiration. The fundamental reference, determined because the practice is to place the inscription above the door at the entrance to your house, seemed to me to be to St Matthew's Gospel (Mt 2:10-11, with my emphasis added):
When they saw the star, they rejoiced exceedingly with great joy; and going into the house they saw the child with Mary his mother, and they fell down and worshipped him.One sense of this custom, then, is that every time we enter our own house we are reminded of the way in which the Magi entered the house and worshipped the child Jesus. A second sense is that we are encouraged to make our homes places where, like Mary, we worship God. We might also see it as an invitation to guests who visit our house to recognise and share in these two senses of the inscription. This seems to me the simple, straightforward sense of the practice based in its Biblical source.
But what interests me for the purposes of this post is how the idea of "the house" in the Gospel passage (the house where Mary and Jesus were staying) becomes a different idea (the house were we live today) in the custom of marking our door frames with the letters "C+M+B".
When it comes to considering the new English translation of the Domine, non sum dignus in the Communion Rite at Mass, I think that it is helpful to recognise a similar way in which a Gospel text is used. The new translation makes much more transparent than did the previous translation the reference to the Gospels (Mt. 8:8, with a parallel in Lk. 7:1-10):
Lord, I am not worthy to have you come under my roof; but only say the word, and my servant will be healed.The "under my roof" of the Gospel text (ie the house of the Centurion whose servant is ill, and to which Jesus has been invited) becomes, in the Liturgical text, the "under my roof" of the physical body and soul of the person about to receive the Eucharist, now seen as a "house" to which Jesus-Eucharist is invited. The act of a physical healing becomes the idea of a saving from sin through receiving the Eucharist. Again, I think the Biblical text itself should be allowed to speak to our understanding of how it is used and becomes a different idea in its Liturgical use.
In the light of Andrew Cameron-Mowat SJ's criticism of the revised translation on this point at Thinking Faith, I think it is useful to have some understanding of the dynamic of how the Biblical text is used in the Liturgical context. Since, in the Gospel text, Jesus has indicated that he will come to the house of the Centurion in the immediately preceding verse, the suggestion that the Liturgical text is intended for a situation where the faithful do not as a rule receive the Eucharist seems to me a reading into the text of something that is not there. To go on to argue that the previous translation - "Lord, I am not worthy to receive you .." - provides a better reading for times when many more people do receive the Eucharist is in consequence irrelevant.