And he said,
I will make all my goodness pass before you, and I will proclaim the name of the LORD before you; and will be gracious to whom I will be gracious, and will show mercy on whom I will show mercy.
Read Chapter 33
Augustine of Hippo
So if the whole world was being detained in captivity, it was quite in order to say, “I will be merciful to whom I will be merciful and show mercy to whom I will show mercy.” If the whole world is in captivity, the whole world in sin, the whole world very justly sentenced to punishment, but part of it set free through mercy, who can say to God, “Why do you condemn the world?” How can God, the just judge, be indicted when the guilty world is convicted? You’re guilty. If you consider what you owe, it is called punishment, and you cannot in fairness blame the one who inflicts it for exacting from you what you owe. You may blame the debt collector if he seizes what you don’t owe, but who can blame a creditor for demanding payment of a debt, even though you are hoping he will let you off? Sermon
What did he here teach us but that as death is the just due of the clay of the first man, it belongs to the mercy of God and not to the merits of man that anyone is saved. And … therein there is no injustice with God, because he is not unjust either in forgiving or in exacting the penalty. Mercy is free where just vengeance could be taken. From this it is more clearly shown what a great benefit is conferred on the one who is delivered from a just penalty and freely justified, while another, equally guilty, is punished without injustice on the part of the avenger.
Now here please note carefully what I am to say, because of the Jews. For it is our purpose to demonstrate that the Lord, Jesus Christ, was with the Father. The Lord then said to Moses, “I will make all my beauty pass before you, and in your presence I will pronounce my name, ‘Lord.’ ” Being himself the Lord, what Lord does he proclaim? You see how in a veiled manner he was teaching the holy doctrine of Father and Son. Again, in what follows, it is written in express terms: “Having come down in a cloud, the Lord stood with him there and proclaimed his name, ‘Lord.’ Thus the Lord passed before him and cried out, ‘The Lord, the Lord, merciful and gracious, slow to anger and rich in kindness and fidelity, and guarding justice and continuing his kindness for a thousand generations, and forgiving wickedness and crime and sin.’ ” And thereafter: “Moses at once bowed down to the ground in worship” before the Lord proclaiming the Father, and said, “O Lord, do come along in our company.”
All good, that could reasonably be desired. "I will pass before thee in all my glory "(Septuagint) and principally in my beneficence, chap. xxxiv. 6, 7. (Calmet)
I will show thee what great favours I have in reserve for Israel. Divines dispute whether Moses saw the divine essence. (St. Thomas Aquinas, 1. p, q. 12, a. 11.) (Menochius)
If he requested to do so now, it seems to be denied, ver. 20. and John i. 17. (Tirinus)
Proclaim When I pass, I will repeat some of my glorious titles, and particularly that I am merciful. (Menochius)
Yet I will show mercy with discretion, and will punish some of you. (Calmet)