It all comes down to nature and grace. If your understanding of nature and grace is wrong it will affect your entire theology. Case in point is the Protestant doctrine of “total depravity.” This doctrine leads them to have a fundamentally wrong understanding of Soteriology (among other things). God does not just declare us righteous as a judge in the courtroom. We do not gain a mere legal status. Rather, when God declares us righteous, we actually become righteous. We do not gain a legal status, but adoption into the Familia Dei, the Family of God. Because of our baptism, we can truly call God “Father” as Jesus taught us to do. We must always remember that grace does not destroy nature, but elevates, heals and perfects it. The Protestant understanding of imputed righteousness denies the power of God’s grace. The logical consequence of the doctrine says that God is not powerful enough to bring about an ontological change in the justified man (not that the nature itself changes, a justified man is still a man, but his nature is perfected in Christ. The Protestant understanding would not have it so). Here is a great summation of nature and grace by John Haas:
“The saeculum, or the natural order, is distinct from the spiritual order but it is not separated from it and is certainly not opposed to it. One of the errors that arouse in much Protestant thought, and persists to our own day even in secular culture, is that the natural and supernatural orders are opposed to one another. Because of the doctrine of the total depravity of man, classical Protestantism tends to look at fallen man as radically over against God. In the classical Protestant theory of justification, man is never truly made righteous. God merely regards him as righteous by virtue of the saving actions of Jesus Christ performed on his behalf. Man is treated by God only as though he were righteous, that is, he has righteousness imputed to him. Man remains a depraved sinner even as he is justified, which is the Protestant doctrine of simul Justus et peccator, that is, one is justified, treated as just, even though he remains a sinner.
As a result of this erroneous thinking, there has developed in the general Protestant/secular culture the perception that nature and grace, nature and supernature, the profane and the sacred, the secular and the religious are opposed to one another. While this mistaken thinking has profoundly influenced even some Catholic thinkers, it is a notion that is profoundly un-Catholic. Even though an infinite gap separates the Creator from the creature, as articulated in the thought of Saint Thomas, they are not in opposition to one another.
The saeculum, or the natural order, is distinct from the spiritual, but it is not separated from it. In the hierarchy of being, the natural order is ordered toward the supernatural so that it never loses any of its own proper, distinct, and unique created essence, derived from the Creator itself.”
-John M. Haas, “The Relationship of Nature and Grace in St. Thomas Aquinas,” in The Ever-Illuminating Wisdom of St. Thomas Aquinas.