According to Catholics, was faith in the assertions made in the Creed of St. Athanasius required of those in the first 3 centuries? If the person that penned the Creed of St. Athanasius that was identified by St. Thomas Aquinas as being St. Athanasius based on "style" had not published his opus and had it declared Truth until the Fourth (4th) Century AD; and

One must believe every word of it or burn in Hell forever; and

No one before St. Thomas Aquinas (1225-1274) had heard of this information.

Should we understand there to have been no Christians prior to the fourth (4th) century AD?

If not, why not?
It seems you're asking about the difference between a material (definable) and formal (defined) dogma.


material (definable) dogma: a truth revealed by God that the Church has not yet proposed one has to believe under pain of mortal sin against faith and excommunication
formal (defined) dogma: "A truth proposed by the Church as revealed by God"*


Before the 4th century, the Athanasian Creed was a material dogma; no Christian is bound, under pain of mortal sin and excommunication, to assent to a material dogma, to a creed that did not yet exist.

The Athanasian Creed's "way of speaking" ("style") is not what makes it formal dogma; that only makes it definable. What makes it "A truth proposed by the Church as revealed by God"* is that "it was accepted by the authority of the Sovereign Pontiff, so as to be considered as a rule of faith" (Summa Theologica II-II q. 1 a. 10 ad 3).

*On the Value of Theological Notes and the Criteria for Discerning Them by Father Sixtus Cartechini S.J. (Rome, 1951)

Your question seems to be about how Christian doctrine develops yet remains always the same. The development of doctrine is analogous to the development of a living organism, which grows yet always remains the same. See the related question "Whether the articles of faith have increased in course of time?" (Summa Theologica II-II q. 1 a. 7) co.:


  The articles of faith stand in the same relation to the doctrine of faith, as self-evident principles to a teaching based on natural reason. Among these principles there is a certain order, so that some are contained implicitly in others; thus all principles are reduced, as to their first principle, to this one: "The same thing cannot be affirmed and denied at the same time," as the Philosopher states (Metaph. iv, text. 9). In like manner all the articles are contained implicitly in certain primary matters of faith, such as God's existence, and His providence over the salvation of man, according to Heb. 11: "He that cometh to God, must believe that He is, and is a rewarder to them that seek Him." For the existence of God includes all that we believe to exist in God eternally, and in these our happiness consists; while belief in His providence includes all those things which God dispenses in time, for man's salvation, and which are the way to that happiness: and in this way, again, some of those articles which follow from these are contained in others: thus faith in the Redemption of mankind includes belief in the Incarnation of Christ, His Passion and so forth.
  
  Accordingly we must conclude that, as regards the substance of the articles of faith, they have not received any increase as time went on: since whatever those who lived later have believed, was contained, albeit implicitly, in the faith of those Fathers who preceded them. But there was an increase in the number of articles believed explicitly, since to those who lived in later times some were known explicitly which were not known explicitly by those who lived before them. Hence the Lord said to Moses (Ex. 6:2,3): "I am the God of Abraham, the God of Isaac, the God of Jacob [*Vulg.: 'I am the Lord that appeared to Abraham, to Isaac, and to Jacob'] . . . and My name Adonai I did not show them": David also said (Ps. 118:100): "I have had understanding above ancients": and the Apostle says (Eph. 3:5) that the mystery of Christ, "in other generations was not known, as it is now revealed to His holy apostles and prophets."