Justin I'm familiar with.
I always chuckle inside when I hear someone saying that the doctrine of the Trinity originated in 325 AD at Nicea. Sometimes they even suggest that it was only after Christianity fell under the sway of evil emperor Constantine that anyone ever thought that Jesus was God and that the whole thing was politically motivated. While it is true that some of the more developed Trinitarian language and technical distinctions were worked out much later, after centuries of careful reflection and debate, the basic outlines of Trinitarian doctrine are in the pages of the New Testament. Scripture set the axioms, and as Christians talked, the logical requirements of these axioms became clearer and the theological language necessary to express these requirements began to develop.
Jesus is God. At several points, Justin lists names that the scriptures, as he reads them, give to the Messiah. He is called an “angel,” for instance, because he is brings a message from God (ἄγγελος = messenger) and “Lord” because of his authority. Along with these titles, Justin insists that the Messiah is God: “Permit me, further, to show you from the book of Exodus how this same One, who is both Angel, and God, and Lord, and man…” (§59. Cf. §§60,61,127,129). We can notice throughout that Justin is not content to say that the Messiah is merely “divine,” but rather insists that he is the “Son who is God (Θεὸν ὄντα Ὑιόν)” (§127).
source: Justin Martyr on the Trinity
The question is:
Did he know what the creeds declare?
Other ECFs did....exactly what each one believed would require much study.
The Creeds say nothing about their being two Gods. The Creeds also go beyond the subordinationism of Justin and others.
I do believe that saying Jesus is the Messiah and stopping at that would not be sufficient.
Peter added that Jesus is the Son of the living God. That defines the Messiah for me.