As a convert from Protestantism, one of the challenges was the concept of grace. It was taught, and still is, that justification and sanctification were an instantaneous barrage of grace that instantly transformed. Granted, this is more of a Baptist, view and can change based on denomination. This differs widely from the Catholic view that grace more of a renovation for the soul.
Protestant theology looks at grace as a forensic, or declarative, justification. The reformers saw the concept of concupiscence and took it a step further and said that man is totally depraved. This total depravity prevents man from doing anything good, and all good things done are done by God. Therefore, man is unable to do good even with the help of sanctifying grace. The work of Christ on the cross is therefore imputed to the sinners account when a faith in Christ is declared. Fr. John Hardon writes that in the Protestant view a sinner is “righteous by reason of the imputed merits of Christ and a sinner because his inherited guilt remains” (Hardon Ch.4). Justification is now a matter of declaration whereby the righteousness of Christ is imputed to the sinner making the Father see the sinner as righteous.
The is in stark contrast to the Catholic view that sees sanctifying grace as a transformative force that changes the sinner into a saint. The journey takes a lifetime and is fills with highs and lows. Through the sacrament of baptism all sin is washed away, and we have a clean slate. Through the voluntary of grace on a daily basis and is ours. Regarding this John Hardon states, “what we obtain is truly ours and no mere judicial attribution” (Hardon Ch.4). It is given to us to transform us, not merely to make a once time declaration and not change our nature. In the Protestant system we are not changed, and in the Catholic system Christ transforms us.
Hardon, John. History and Theology of Grace. Ann Arbor, MI: Sapientia Press, 2005.