St. Gregory the Great

The gold, incense and myrrh we may interpret in yet another way. For by gold wisdom is symbolized, as Solomon testifies: Desirable treasure lies in the mouth of wisdom (Prov. 21:10). By incense, which is burnt in honor of God, we signify the power of prayer, as the Psalmist testifies who says: Let my prayer be set forth as incense before Thee (Ps.140:2). By myrrh is symbolized the mortification of our flesh; whence it is that the Holy Church says of her workers who have labored for God until death: My hands dripped with myrrh (Song of Songs 5:5). To the newborn King we offer gold, if we shine before Him in the brightness of heavenly wisdom. We offer Him incense, if we consume the thoughts of the flesh upon the altar of the heart, so that in our heavenly desires we send up to God an odor of sweetness. We offer Him myrrh when we mortify through abstinence the vices of our flesh.” - Saint Gregory the Great