From his Elevations on the Mysteries, XV-IV:
His name will be Emmanuel: God with us. These are the mystical names which the Prophets give in spirit, to explain certain effects of divine power, which are necessary for those who use them. If we understand the force of that name Emmanuel, we find that of the Saviour. For what is it to be a saviour, if it is not to take away sins, as the angel had explained? But the sins being removed, and having no more separation between God and us, what else is left, unless being united with God and that God is with us perfectly? We are thus perfectly and eternally saved, and we recognise in Jesus who saves us, a true Emmanuel. He is Saviour, because in him, God is with us; it’s a God who unites our nature; reconciled with God, we are lifted up by his grace, until we are no more than a same spirit with him.
It is he which works who is at the same time God and who we are: God and man all together. God is in Jesus Christ reconciling himself to the world, not imputing to them their sins, and wiping them away in the Saints. Thus God is with them, because they no longer have their sins.
But this would be nothing if, at the same time, God was not with them to prevent their committing new ones. God is with you, in the style of the Scriptures, that is to say that God protects you, helps you, and then with a help so powerful that your enemies cannot prevail against you. They fight, says the Prophet, and they do not prevail, because I am with you. (Jer. 1,19) Be with us, O Emmanuel, so that if, after the forgiveness of our sins, we fight his pernicious enticements, his personality, his temptations, and we stay victorious.
Is this all the grace of our Emmanuel? Doubtless no; in him there is a good much higher which also the last of all; it is that he will be with us in eternity, where God will be all in all, with us for us to purify our sins, with us to sin no more, with us for us, to the life where we cannot commit more. See here, says St. Augustine, three degrees by which we pass to come to the salvation which the name of Jesus promises us and the perfect grace of the divine union by our Emmanuel: happy, when not only we do not sin any more under the yoke of him to whom we succumb, but when we no longer have to come against him whom we have had to fight, and who put our deliverance at risk.
O Jesus! O Emmanuel! O Saviour! O God with us! O conqueror of sin! O bond of the divine union! I wait with faith for that happy day where you will receive for me the name of Jesus; where you will be my Emmanuel, always with me, among all the temptations and dangers; go before me with your grace, unite me with you, and may all that is within me be submitted to your will.