Aquninine determinsim, itself essentially Augustinian in nature, was rejected as heterodox by the Catholic Church, and remains so today. Interestingly, Aquinas also proposed that beings are capable of independent agency seperate from God's direct control. From my persepctive, it seems that every attempt to remove Free Will from the Christian worldview is born of angst over relatively minor questions regarding how God's own nature and Divine Knowledge interact with the world he created.

broadly speaking, determinists usually feel the need to explain away one of two logical inconsistancies. The first is that God is all powerful, but that Free Will allows beings to rebel against God's power; the argument is easily undone by stating that God allows rebellion (Sin), but does not condone it. The second argument relates to God's constancy and his Onnicience. While this is harder to answer, it is easy to ignore because, bluntly, the Bible contains numerous instances of rebellion, so it clearly does happen.

On to the more philosophical question:

Why must the exercise of Free Will require reationality, and how do we define who is "rational"? For me, Free Will is apparent because if there were no Free Will or Random Chance the universe would be a perfectly ordered and regular place. It would also be a lot less interesting.