"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 again 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."
Cranmer's collect exhorts us to cast away the works of darkness and put upon the armour of light. These passages are taken from St Paul's Epistle to the Romans Chapter 13 (which is also the Epistle for the first Sunday in Advent in the Old Mass and the Mass of Pope Paul VI). Advent is really a time for penitence and preparation, not just for Christmas but the Parousia.
The BCP prayers and the readings shows a bit of continuity from the Ancient Liturgies of the Catholic Church, received in the Anglican Church which still is being prayed today.