Salvator noster, dilectissimi, hodie natus est: gaudeamus. Neque enim fas est locum esse tristitiae, ubi natalis est vitae: quae consumpto mortalitatis timore, nobis ingerit de promissa aeternitate laetitiam. Nemo ab huius alacritatis participatione secernitur. Una cunctis laetitiae communis est ratio: quia Dominus noster peccati mortisque destructor, sicut nullum a reatu liberum reperit, ita liberandis omnibus venit. Exsultet sanctus, quia appropinquat ad palmam: gaudeat peccator, quia invitatur ad veniam: animetur Gentilis, quia vocatur ad vitam. Dei namque Filius secundum plenitudinem temporis, quam divini consilii inscrutabilis altitudo disposuit, reconciliandam auctori suo naturam generis assumpsit humani, ut inventor mortis diabolus, per ipsam, quam vicerat, vinceretur.
Our Saviour, dearly-beloved, was born today: let us be glad. For there is no proper place for sadness, when we keep the birthday of the Life, which destroys the fear of mortality and brings to us the joy of promised eternity. No one is kept from sharing in this happiness. There is for all one common measure of joy, because as our Lord the destroyer of sin and death finds none free from charge, so is He come to free us all. Let the saint exult in that he draws near to victory. Let the sinner be glad in that he is invited to pardon. Let the gentile take courage in that he is called to life. For the Son of God in the fullness of time which the inscrutable depth of the Divine counsel has determined, has taken on him the nature of man, thereby to reconcile it to its Author: in order that the inventor of death, the devil, might be conquered through that (nature) which he had conquered.
St. Leo the Great, Sermon 1 on the Nativity [Sermon 21 (20), PL 54, 190–191], read as the fourth lesson of Matins in the traditional Roman Breviary.