Objection 1. It would seem that we were not reconciled to God through Christ's Passion. For there is no need of reconciliation between friends. But God always loved us, according to Wisdom 11:25: "Thou lovest all the things that are, and hatest none of the things which Thou hast made." Therefore Christ's Passion did not reconcile us to God.
Objection 2. Further, the same thing cannot be cause and effect: hence grace, which is the cause of meriting, does not come under merit. But God's love is the cause of Christ's Passion, according to John 3:16: "God so loved the world, as to give His only-begotten Son." It does not appear, then, that we were reconciled to God through Christ's Passion, so that He began to love us anew.
Objection 3. Further, Christ's Passion was completed by men slaying Him; and thereby they offended God grievously. Therefore Christ's Passion is rather the cause of wrath than of reconciliation to God.
On the contrary, The Apostle says (Romans 5:10): "We are reconciled to God by the death of His Son."
I answer that, Christ's Passion is in two ways the cause of our reconciliation to God. In the first way, inasmuch as it takes away sin by which men became God's enemies, according to Wisdom 14:9: "To God the wicked and his wickedness are hateful alike"; and Psalm 5:7: "Thou hatest all the workers of iniquity." In another way, inasmuch as it is a most acceptable sacrifice to God. Now it is the proper effect of sacrifice to appease God: just as man likewise overlooks an offense committed against him on account of some pleasing act of homage shown him. Hence it is written (1 Samuel 26:19): "If the Lord stir thee up against me, let Him accept of sacrifice." And in like fashion Christ's voluntary suffering was such a good act that, because of its being found in human nature, God was appeased for every offense of the human race with regard to those who are made one with the crucified Christ in the aforesaid manner (1, ad 4).
Reply to Objection 1. God loves all men as to their nature, which He Himself made; yet He hates them with respect to the crimes they commit against Him, according to Sirach 12:3: "The Highest hateth sinners."
Reply to Objection 2. Christ is not said to have reconciled us with God, as if God had begun anew to love us, since it is written (Jeremiah 31:3): "I have loved thee with an everlasting love"; but because the source of hatred was taken away by Christ's Passion, both through sin being washed away and through compensation being made in the shape of a more pleasing offering.
Reply to Objection 3. As Christ's slayers were men, so also was the Christ slain. Now the charity of the suffering Christ surpassed the wickedness of His slayers. Accordingly Christ's Passion prevailed more in reconciling God to the whole human race than in provoking Him to wrath.
Source: Summa Theologica (New Advent)