The creed explicitly denounces “those who say: 'There was a time when he was not;' and 'He was not before he was made.” In other words, according to the creed, He always existed.
The creed also says that “By whom (Jesus Christ) all things were made both in heaven and on earth.” “All things,” therefore, presumably refers to this entire universe. Space, time and matter defines the universe. By implication, even time was created “by” Jesus Christ, as amazing as that may sound. Again, the conclusion is that He always existed. There was no time that He did not exist.
Θεὸν ἐκ Θεοῦ (theos from theos or god from god), is an interpretation of the description in the Creed of Christ as “begotten” (born). Since He is metaphysically “born” from God, He came out of the being of God and is in a different category relative to created things.
And, as the creed indicates, He is the “only-begotten.” In other words, He is the only one that was ever came out of the being of God.
But then, your question is, how do we reconcile the fact that He always existed with the fact that He was born of God, which implies a beginning? I propose that we explain this by making a distinction between our universe of time, space and matter, and the True Reality, which exists beyond our universe, but from where the power and intelligence came that produced our universe:
Time exists in our universe and Christ existed for all that time, but that time began 13 billion years ago with the big bang.
But in that True but incomprehensible Reality, arguably, time does not exist. In that reality, the Son was born from the being of God, but since time does not exist, there is no before or after and the word “always” does not apply for the word “always” is only valid where time exists.