There is valid point in the inference “If the world is eternal, then it was not created.”
Indeed, the Christian notion of "creation" indicates that the world had a definite beginning. Revelation teaches that the world is not eternal.
Here is how Mortimer Adler explains the Christian notion of "creation."
The word ‘creation’ is used to mean both the causation of being and the definite origin of what is thus caused. It is contradictory to speak of a ‘created eternal world,’ for if created is what has a definite beginning, it cannot also be everlasting or without beginning.
For analytical clarity, it is absolutely necessary to use the word ‘creation’ with these two notes in its signification: (a) to create is to cause being; (b) to create is to cause to begin to be – understanding such ‘beginning,’ of course, as neither a change nor a motion of any kind.
In terms of such verbal usage, there should be no difficulty about understanding what is meant by saying that God can be the cause of being of either an everlasting world or a world with beginning.
(M. J. Adler, “The Demonstration of God’s Existence,” in The Maritain Volume of the Thomist, Sheed and Ward, New York, 1943, pp 188-218.)
How can God be the "cause of being" of an everlasting world?
It is instructive to observe here that, when it is a question of the most radical sense of being, the causation of the beginning of the existence of "something" and the holding of that "something" into existence are inseparable actions springing from one and the same cause.
Evidently an everlasting world does not need to account for its beginning. But how does an everlasting world hold itself into existence?
Here is how Aquinas explains it.
Concerning the "cause of being" one must distinguish two kinds of causation, (1) a causation which is “preservative of being with a beginning in time,” in the sense that there was nothing before the material world was brought into existence; and (2) a causation which is “preservative of being without a beginning in time,” in the sense that the world is eternal and there was never a time when there was nothing in existence.
The first type of causation of being is properly speaking creation. The second type of causation means only conservation of being.
Note on Translation: The expression "actus essendi" is translated into English as "act of being," into Italian as "atto di essere," into French as "acte d'être," into Spanish as "acto de ser," and into German as "Akt des Seins" ("Seinsakt.")