Almighty God, give us grace that we may cast away the works of darkness,
and put upon us the armour of light, now in the time of this mortal life,
in which thy Son Jesus Christ came to visit us in great humility; that in
the last day when he shall come in his glorious majesty to judge both the
quick and the dead, we may rise to the life immortal, through him who liveth
and reigneth with thee and the Holy Ghost, now and ever. Amen.
The 'Advent Collect' (as it is popularly called) was composed for the 1549 Book. In the Latin Missal the Collect for this Sunday is a 'stir up' Collect similar to the ones for the Fourth Sunday and the Sunday next before Advent. The rubrical direction to repeat it every day during the season first appeared in the 1662 Book, at the suggestion of Bishop Matthew Wren. There is nothing in the Sarum or Roman Missals comparable to the Prayer Book custom of a seasonal Collect, as we have during Advent and Lent.
The Collect is based upon verse 12 in the Epistle for this Sunday (Rom. viii. 8). It is remarkable for its striking antitheses: cast off darkness--put on light; now--in the last day; mortal life--life immortal; great humility--glorious majesty. The crucial word is 'now,' which ties together the whole--past, present, and future. It should be noted, moreover, that the 'now' does not refer to the time or season of Christ's coming, but to our own immediate time (cf. the verse in the Epistle, 'now it is high time to awake ...'); for 'in which' modifies 'this mortal life' not 'in the time.' The word 'visit' is used in this Collect in the sense of 'visitation' (cf. Luke xix.44). The description of the Incarnation in terms of 'great humility' reminds us of the use of the same phrase by Cranmer to describe our Lord's Atonement (in the Collect for Palm Sunday). (For the 'armour of light,' cf. Eph. vi.13-17.)
from The Oxford American Prayer Book Commentary by Massey Hamilton Shepherd, Jr. (New York : Oxford University Press, 1950)