After his fall, man was not abandoned by God. On the contrary, God calls him and in a mysterious way heralds the coming victory over evil and his restoration from his fall. This passage in Genesis is called the Protoevangelium (“first gospel”): the first announcement of the Messiah and Redeemer, of a battle between the serpent and the Woman, and of the final victory of a descendant of hers.
The Christian tradition sees in this passage an announcement of the “New Adam” who, because he “became obedient unto death, even death on a cross”, makes amends superabundantly for the disobedience, of Adam. Furthermore many Fathers and Doctors of the Church have seen the woman announced in the “Proto-evangelium” as Mary, the mother of Christ, the “new Eve”. Mary benefited first of all and uniquely from Christ’s victory over sin: she was preserved from all stain of original sin and by a special grace of God committed no sin of any kind during her whole earthly life.
But why did God not prevent the first man from sinning? St. Leo the Great responds, “Christ’s inexpressible grace gave us blessings better than those the demon’s envy had taken away.” and St. Thomas Aquinas wrote, “There is nothing to prevent human nature’s being raised up to something greater, even after sin; God permits evil in order to draw forth some greater good. Thus St. Paul says, ‘Where sin increased, grace abounded all the more’; and the Exsultet sings, ‘O happy fault,. . . which gained for us so great a Redeemer!'”
From the Catechism of the Catholic Church 410-412