Catholic theology is so deep that I find myself getting lost and revisiting the basic tenets of the faith. So I need some help here.
Mary was predestined to be sinless.
Sin is an act of the will, she chose to remain sinless out of free will.
It was an increase of God’s grace in her that enabled her to be sinless.
Can someone tear these three things apart and piece them together slowly to help me understand how the three - predestination, grace, and free will are compatible here.
And also help me understand why it was possible for Mary and not for others? Is it possible for others also?
The relationship between predestination, grace, and free will is a very old and very difficult subject. A Catholic can safely say we believe in all three, but exactly what they mean and how they connect to each other is highly controversial, and you will find a lot of diversity among Catholics on the subject, and even diversity on the issue of how much diversity is allowed.
On the one hand my view is that in the current Magisterial climate any theory on the subject that has not been expressly condemned and which does not deny any of those three elements wholesale seems to be permitted, and we should defend the right of Catholics not to be called or implied to be heretics just because they have come to a different theological conclusion than someone else has.
On the other hand I do have a view of my own, which combines different elements of the two major Catholic schools of thought on this subject that are often brought up, the Thomist school and the Molinist school (and some elements that wouldn’t fit neatly into either). I’m not entirely sure about that theory though, because I’m not totally confident about my own grasp on the subject and my own intelligence.
In my tentative view the basic outline of the connections between your three elements, in your example of Mary, would be that God gave Mary special graces which, as an adult, she could have freely accepted or rejected, but she freely chose to always accept that grace because she had been predestined to do so by God. Thus you’d have God’s sovereign plan of predestination over all, with the giving of grace as something outside Mary’s control and the acceptance of that grace as something in her control in one sense but still subsidiary to God’s plan that she accept it.
This idea a great many Catholics would reject and hold instead that God’s “predestination” of Mary’s acceptance of grace comes logically
subsequent to, not logically
prior to God’s knowledge that Mary will in fact accept that grace.