There is a pious custom found in the Roman Ritual which consists of blessing houses on the day of Epiphany. For this, we read at the entrance of the house the Prologue to the Gospel of Saint John, we say an Our Father, we read the Collect of the day of the Epiphany, and finally, we make the inscription above the front door with the chalk blessed by the priest.

The inscription for 2021 is :

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The numbers show the year in which the inscription is made.

The letters have two meanings:

They are the initials of the names of the three Three Kings:

Gaspar (Caspar), Melchior and Baltazar.

They are also the abbreviation of Latin words

Christus mansionem benedicat. May Christ bless this house.

The crosses represent the protection of the Precious Blood of Christ, whose sacred name we invoke, and also the holiness of the three Magi, sanctified by their adoration of the infant Christ.

The inscription is made above the front door of the house (and not above the doors of the different rooms of the house as some do), so that all who enter and leave the house house during the year can receive God's blessing. 

The inscription is made with chalk, a product of clay, which recalls the human nature taken by the Adorable and Eternal Word of God in the womb of the Virgin Mary, by the power of the Holy Ghost.

To bless your home on this day of Epiphany, first read the Prologue to the Gospel of Saint John (see below), followed by the Our Father and the Collect of the Epiphany (see below). below); then, write the inscription for that year above your front door with the blessed chalk.

Prologue to the Gospel of Saint John

In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by Him, and without Him was made nothing that was made: in Him was life, and the life was the Light of men; and the Light shineth in darkness, and the darkness did not comprehend it.
There was a man sent from God, whose name was John. This man came for a witness, to testify concerning the Light, that all might believe through Him. He was not the Light, but he was to testify concerning the Light.
That was the true Light, which enlighteneth every man that cometh into this world. He was in the world, and the world was made by Him, and the world knew Him not. He came unto His own, and His own received Him not. But as many as received Him to them He gave power to become sons of God, to them that believe in His Name, who are born not of blood, nor of the will of the flesh, nor of the will of man, but of God. And the Word was made flesh, and dwelt among us: and we saw His glory, the glory as of the Only begotten of the Father, full of grace and truth.

Our Father.

Collect of the Epiphany

O God, Thou Who by the guidance of a star didst this day revealed Thine only-begotten Son to the Gentiles; mercifully grant that we who know Thee now by faith, may come to behold Thee in glory. Through the same Jesus Christ, Our Lord. Amen.

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