“In the old liturgy for Palm Sunday, the priest, arriving in front of the church, would knock loudly with the shaft of the processional cross on the door that was still closed; thereupon, it would be opened. This was a beautiful image of the mystery of Jesus Christ himself who, with the wood of his Cross, with the power of his love that is given, knocked from the side of the world at God's door; on the side of a world that was not able to find access to God. With his Cross, Jesus opened God's door, the door between God and men. Now it is open."
-- Homily of Benedict XVI, Palm Sunday, 2007
(Liturgical Arts Journal)