The Blessed Virgin Mary was preserved from original sin by a singular grace and privilege of God at her Immaculate Conception. She was conceived in the state of grace, and was unfallen in body and soul. Mary was also preserved from all personal sin by the grace of God. So she never sinned at all, in any way, to any extent, in her entire life.
The question — as a pure hypothetical, which certainly did not occur — is this: Was the Blessed Virgin Mary able to sin, if she would have chosen to do so? In other words, was Mary incapable of sin, or did she simply choose not to sin? There are two answers to this question:
1. that Mary could have sinned, if she so chose, but she never did choose to sin, by cooperation with grace
2. that Mary was absolutely prevented from sinning at all, by the prevenient grace of God
Consider the case of Adam and Eve, prior to the fall from grace. Adam and Eve were created in the state of grace. They began with no original sin and no personal sin. Yet they both sinned by free will, despite grace. Is Mary like Eve, except that she did not choose to sin?
The problem with the first answer is that the plan of salvation becomes subject to Mary’s will.
Any sin committed by the Blessed Virgin Mary (as a pure hypothetical) would have to be a mortal sin. Like Eve, she was not capable of venial sin. For any sin by Eve meant that she would lose the state of original innocence, which is a grave consequence, making any sin mortal in gravity. And the same is true for Mary. Any sin committed by her would have caused her to lose the gift of grace given at her Immaculate Conception, and that would be a very grave sin. So if she could have chosen to sin, her first sin would be mortal, not venial.
And if Mary committed a mortal sin, she would no longer be fit to be the mother of God. This change would have disrupted God’s plan of salvation. Being all-powerful, God could have then saved us in a different way. But it seems absurd for God to have a perfect plan, and then to allow it to be disrupted by the will of Mary.
We could say that God simply foresaw that Mary would never choose to sin, even though she could have sinned. But this plan, too, despite its obvious success, is still reliant on the will of Mary. It is as if we are only saved, by this wonderful plan, because Mary somewhat capriciously chose not to sin, and God foresaw that choice. God’s plan then becomes subservient to Mary.
Another consideration is that the choice to refrain from sin is not one decision, but many decisions every day, for many days, months, and years. So, if Mary could have chosen to sin, it seems foolish to build the plan of salvation on a collection of very many choices not to sin. It is like a house of cards or a set of dominos. The whole house falls or all the dominos collapse, if just one falls. That is not a fitting plan to save billions of souls.
But does God have any other option, given that He chose to give us all free will? The fundamental question here is whether the grace of God can, or ever does, absolutely prevent free will from choosing to sin. If the answer is “No,” then the first solution to the problem prevails. But if the answer is “Yes,” then the second is most fitting.
Are there any cases where God absolutely prevents persons with free will from sinning? Yes, very many cases. All the souls and angels in Heaven are entirely unable to sin. They have the Beatific Vision of God, and so they know God, who is Truth and Love. That is one reason that they do not sin. But another reason is that they are each filled with grace, and that grace prevents even the possibility of sin.
There are two types of grace: prevenient grace (God operating) and subsequent grace (God cooperating). Prevenient grace is described by Saint Thomas in the Summa Theologica, and is taught by the Council of Trent. Prevenient grace is also mentioned in the Catechism of the Catholic Church. It is the teaching of the Church that some graces of God act prior to even the possibility of acceptance (by cooperation) or rejection of that grace.
Prevenient grace prevents the souls and angels in Heaven from sinning. Now some persons will object to this assertion, saying that it is the Beatific Vision that prevents sin in Heaven. My reply is that all the souls in Purgatory also cannot sin, otherwise some of them would never enter Heaven. But the doctrine is that all the souls of Purgatory eventually go to Heaven. And they have no Beatific Vision in Purgatory. So what prevents them even from the possibility of sin is prevenient grace. And this grace is fair to us, for it is part of our lot in life, as creatures of God, to be subject to some decisions by God that are beyond our control. The souls in Purgatory chose the path to Heaven, and sinlessness is the result.
But what about wicked persons? Prevenient grace also prevents the souls and devils in Hell from continuing to sin gravely against God forever — a situation which would be contrary to Divine Justice. They chose the path to Hell by their grave sins, and therefore they are subject to the situation in Hell, which is that they can no longer sin at all. They are not in a state of grace, so this freedom from sin is not due to cooperation with grace. It is due solely to prevenient grace, which forbids them from continuing to sin forever.
Thus, we see that God can prevent a person from sinning, despite free will, regardless of whether that person is very holy (souls and angels in Heaven), or sinful but in a state of grace (the souls in Purgatory), or wicked (the souls and devils in Hell). Therefore, it is possible and entirely fitting that God prevent the Blessed Virgin Mary from ever committing any sin, so as to secure the path of salvation for us all and so as to prepare her as a fitting Mother for the Son of God.
By the way, this prevenient grace also affects Peter and each and every successor of Peter, preventing each from ever teaching heresy, and from ever committing the sins of apostasy, heresy, or schism.
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