Aquinas' debate about God's omnipotence is found in his Summa Theologica. This is a vast work which covers a huge range of theological and philosophical issues.
The structure of the Summa Theologica is somewhat like a discussion or debate, but without the drama of Plato's Dialogues. Aquinas began each topic by first posing a question (e.g. 'Whether God is omnipotent?'). Next he presented 'objections' to the idea. These objections might be scriptural, theological or philosophical in nature. After the objections he would write 'on the contrary' and then giving a piece of counter evidence to the objection. Aquinas' own view comes next prefaced with 'I answer that...'. Finally he systematically refutes or answers the objections that he previously set out.
Whether God is omnipotent?
Objection 1: He cannot be because he is immovable [has no potential to change because he is fully actual already].
Objection 2: God cannot sin.
Objection 3: According to the collect (special prayer) for the tenth Sunday after Pentecost, God shows his omnipotence 'especially by sparing and having mercy'. However, there are things greater than these [so if this is the pinnacle of God's power then he is not very powerful].
Objection 4: The Bible says that God makes worldly wisdom look foolish by showing that what is thought to be impossible possible.
Thus the standard for knowing what is possible or impossible must come from God and not from the world.
If God were omnipotent then he would be able to do anything.
Thus there would be nothing that is impossible.
However, abolishing impossibility also abolishes necessity because something is necessary if it is impossible that is should not be so.
On the contrary: It is said that "No word shall be impossible with God" (Lk. 1:37)
I answer that, All confess that God is omnipotent; but it seems difficult to explain in what His omnipotence precisely consists of.
Aquinas recognised that 'omnipotent' can be interpreted in different ways. He went on to explain that in his view it must mean that God can do all possible things. However, this does not yet give us complete clarity. What do we mean by 'possible'? Like the word 'omnipotent' both 'possible' and 'impossible' can have different shades of meaning.
According to Aquinas, when we talk of a thing being 'possible' we mean one of two things.
Thus, when we speak of something being 'possible' we mean either
If we use the first definition of 'possible' (that it is within an agent's power) then this does not tell us anything very meaningful about God's power. God can do all humans can do and more. He can do everything that all created beings can do. Everything is within God's power and God can do whatever is within his power to do. However, Aquinas pointed out that this because a tautology 'God can do all that he can do'.
The second definition of possible is more helpful. He can 'do all things which are absolutely possible.'But God cannot do something when'the predicate is altogether incompatible with the subject.' (such as a human being a donkey).
Reply to Objection 1: God is omnipotent due to what he can do rather than what he cannot do. God's changelessness does not undermine his omnipotence.
Reply to Objection 2: To sin is to fall short of what should be achieved, to fall short is to fail to be omnipotent. Thus it is God's omnipotence is what prevents him from sinning and the inability to sin is not a lack.
Aquinas further suggests that God could sin if he chose to do so. However, this is a conditional proposition (IF he chose to do so THEN he could sin). The fact that God's omnibenevolent nature is such that he would never choose to do so means that in practise he would be incapable sin. Aquinas suggests the following analogy to make the point. 'If man is a donkey, he has four feet.' The fact that man is not a donkey obviously means that in actual fact he does not have four feet.
Reply to Objection 3: God's sparing and having mercy especially reveals God's omnipotence because it is a free act that shows God's supreme power. It is only because God is bound by no laws and has no superior that he is free to forgive and show mercy. God's mercy also leads people to do good which is the end point of all of God's power. Everything comes from God and thus God's sparing and having mercy is the foundation of everything else.
Reply to Objection 4: