God said he was “I am who am”. This is the act of existing. It is a necessary attribute of the first cause. The act of existing cannot be created as it exists eternally.
The rest of the argument is invalid as it is based on the false premise that the act of existing must be created.
Of course the Act of Existing which is God is not created. But the " act of existing " of creatures is created.
God does not give created beings His being - or a bit of it so as to say that they " share " in God’s own existence. That is Pantheism. It is also a violation of the Principle of Contradiction, for a thing cannot be and not be in the same respect at the same time. So if their being is God’s being, they are not real created beings, they are " One " with God. And that is Pantheism.
As Thomas says in S.T. 1, 45, 4 " God is the cause of created substances, which includes existence. And to be created, means also to be absolutely other than God, even in their " act of existence. "
Again, in 45, 5 he states, " Among all effects the most universal is existence itself…"
and further on he says, " Now God’s proper effect is that which is presupposed to any other, namely existence
tout court ( simply ). "
And again in S.C.G., Book 1, ch 26 Thomas says: "… Chapter 26
THAT GOD IS NOT THE FORMAL BEING OF ALL THINGS
[1] We are now able to refute the error of certain persons who said that God is nothing other than the formal being of each thing.
[2] This being is divided into the being of substance and the being of accident. Now, we have proved that the divine being is neither the being of substance nor that of accident. God, therefore, cannot be that being by which each thing formally is…"
dhspriory.org/thomas/ContraGentiles1.htm#26
Or again, in 1 Sentences, Distinction 37, Ques 1, Thomas says : " Solution: I answer that it should be said that God is essentially in all things, not nonetheless so that he is mixed together with things as if he were a part of any thing…"
www4.desales.edu/~philtheo/lo…ntd37q1a1.html
So when Thomas says created beings " share " in existence, he means they share in the common note or perfection of existence God has given to each created extent. They are said to " share " or " participate, " not because they share in God’s own Existence, but becuase they each have a limited or imperfect existence, which is the same type of " act " that each other created being has been given, but differing in its perfection, as determined by the limiting potency of their respective forms.
Now since they each have the same perfection, differing only in perfection, none can have caused that perfection. It can only be caused by that Being who is Perfect Existence. But this applies to their entire substances and all that goes into the make up of their substances, includling matter, form, and existence, their whole and entire being. They " share " nothing with God. But they do have a real Realation with God, as effects which are caused by Him.
Linus2nd