At the beginning of sacred scripture, we read how God created man. Man was created in a state of grace, and through sin this grace was lost. This led to mankind having the stain of original sin, and a desire to sin called concupiscence. This nature requires grace to assist us in our post-lapsarian nature. This is done because grace effects the faculty of our mind and the capacity of our will.
A result of sin is that we focus on carnal things. No matter how hard we try to avoid sin we will fall back into it without the help of grace. Our minds can be restored through grace, and through this grace we have a greater propensity to avoid mortal sin (ST II, Q 109, A 8). According to Aquinas, grace transforms the mind and makes one alert to situations that will make us fall from grace. It helps us know what is good, and what we should and should not do. We know what we should do, are in a state of grace, and can ask God to assist us in doing the right thing. Grace can effect the faculty of the mind by helping us avoid mortal sin, though we may still commit venial sin (ST II, Q 109, A 8).
Grace also has a strong effect on the capacity of our will. Regarding the will in post-lapsarian man Fr. John Hardon writes, “because of the fall the moral will is a passive faculty which always leans on the side where the weight of attraction is stronger” (Hardon Ch. 3). Our wills strive to make contact with things, and our sinful nature will always go toward the greatest attraction. Grace comes in and alters what attracts us. Through grace our will strives to love God and others. The light of grace turns a selfish will towards hope and charity, and through hope and charity we can see the love of God (Journet 1.6). We can love God and in turn reflect that love toward others.
Aquinas, Thomas. Summa Theologia. Trans. Thomas Gornall. Blackfriars, St. Joseph, IN: Ave Maria Press, 1981. Accessed August 10, 2018.
Hardon, John. History and Theology of Grace. Ann Arbor, MI: Sapientia Press, 2005.
Journet, Charles. The Meaning of Grace. Princeton: Scepter Publishers, 1997.