Can you explain this a bit more. I'm not sure if I agree or disagree.
I did not want to dig through the Summa for the formal arguments; but Wiki had a fairly good synopsis
here:
"Aquinas also understands God as the transcendent cause of the universe, the "first Cause of all things, exceeding all things caused by Him," the source of all creaturely being and the cause of every other cause. Consequently, God's causality is not like the causality of any other causes (all other causes are "secondary causes"), because He is the transcendent source of all being, causing and sustaining every other existing thing at every instant. Consequently, God's causality is never in competition with the causality of creatures; rather, God even causes some things through the causality of creatures.
"
One of the conclusions here could be that our faith is not our own; but is caused by God in all 4 causes.
Material Cause - Our material bodies exist because God choose to create us.
Formal Cause - Our essence is that which God foresaw and willed to be.
Efficient Cause - Well one could say that our beginning started with our parents; but their beginning goes back to their parents, etc back to Adam and Eve.
Final Cause - We know that our purpose is to return to our Creator. That purpose is from God.
So in one sense we are solely God's and God is the primary cause of who we are. The debate begins when we start factoring man's will into this thought versus God's will. If man is a secondary cause of his own purpose, does he have the ability to be the primary cause of his salvation. Or is it all preordained by God?
I think you were using Thomism to mean the total school of St Thomas Aquinas' philosophical thought, while I was reading into the OP's Reformed background to select only the Thomistic views on predestination. I have had discussions with other Reformed Christians on this and some of them see Aquinas as the first to clearly elucidate a philosophy that requires a consideration of predestination and salvation as God's Will through and through. To a Catholic, this contradicts our view on salvation as being a cooperative effort, not a preordained inevitability.