"On earth as it is in heaven"

The Lord's Prayer itself is composed of eight elements, the first of which invokes the Father, the remainder consisting of seven petitions. The first three elements in fact invoke all three persons of the Holy Trinity, as St Maximus explains:

"The words of the Lord's Prayer point out the Father, the Father's name, and the Father's kingdom to help us learn from the source himself to honour, to invoke, and to adore the one Trinity. For the name of God the Father is the only-begotten Son, and the kingdom of God the Father is the Holy Spirit."

If the "name" is the Son, then when the Lord tells Moses (Numbers 6: 22-7) to "call down my name on the sons of Israel" and bless them with the words, "May the Lord let his face shine on you and be gracious to you", he is instructing them
to pray for the coming of Jesus Christ, in whom all the nations of the earth will be saved (cf. Psalm 66).

The third "petition" is given in the form, "Thy will be done on earth as it is in heaven." This, you could say, is the petition of Christmas, when "earth and sky changed places for an hour, And heaven looked upwards in a human face" (G.K. Chesterton).

"God is now on earth, and man in heaven." – St John Chrysostom

In Gentile da Fabiano's bright picture, the child lays his hand on the old man's head, the eternal Child touching the aged earth in order to promise a new spring, indeed a resurrection after death. Heaven reaches down to earth, because in this Child and his mother God's will is done in both earth and heaven for the first time since the Fall. A host of others follow the old man to salvation, wealth kneeling before poverty, earthly wisdom before heavenly foolishness.

As Pope Benedict says in his Christmas homily: "if we want to find the God who appeared as a child, then we must dismount from the high horse of our 'enlightened' reason. We must set aside our false certainties, our intellectual pride, which prevents us from recognizing God’s closeness. We must follow the interior path of Saint Francis – the path leading to that ultimate outward and inward simplicity which enables the heart to see.... Let us allow ourselves to be made simple by the God who reveals himself to the simple of heart."