Ecce Agnus Dei, ecce qui tollit peccata mundi
Behold the Lamb of God, behold him
who taketh away the sin of the world. - John 1:29
"The prayer is made on behalf of those who are about to receive
our Lord's body, that by the mercy of the innocent lamb,
all sinful distractions of word and thought may be driven away, and that
he who once took away sins from all the world may now take them
from the Church, which is to receive Him in the Eucharist." - Amalarius
"As well as by prayer, the people are prepared for the communion
by peace, which is conferred by the recitation of the Agnus Dei.
For this is the sacrament of unity and peace." - St Thomas Aquinas
"Twice we seek mercy, asking that first our soul
and then our body may be freed from misery. And a third time,
we say Agnus Dei, seeking peace for both. - St Antoninus
"The Lamb who is adored in the glory of heaven is present in the Most Holy Sacrament of the Altar here on earth. If a priest were to preach but one sermon from the day of his Ordination until his death, that one sermon could be this: Ecce Agnus Dei, ecce qui tollit peccata mundi; Behold the Lamb of God, behold Him who taketh away the sins of the world (John 1:29).
The priest is but a herald. He announces the presence of the Immolated Lamb, and then annihilates himself in humble adoration. Adoration leads inexorably to self-effacement. The Lamb is exalted; the herald of the Lamb disappears. The Bridegroom shines forth in all His beauty; the friend of the Bridegroom withdraws, content to listen to the sound of his voice.
The priest who adores does on earth what the angels and saints do in heaven. He is employed on earth in the worship of the Lamb that will be his everlasting employment, his rest, and his glory in heaven.
The priest who is not first an adorer has lost the compass that orients all the rest of his life. The priesthood is ordered to adoration, and the summit of adoration is sacrifice: the immolation of a victim to God. The loss of the spirit of adoration is the ruin of the priesthood." - Dom Mark Kirby, OSB