Did Justin Make These Claims Before Constantine and Nicaea?
Justin: 2nd century CE · Constantine: 4th century · Nicaea: 325 CE
What the ancient record says
Justin's Dialogue is placed ~mid-2nd century, roughly a century and a half before Nicaea. This prevents a bad explanation: if someone says "Constantine invented the idea that Gentile Christianoi could claim Yashar'al's writings and identity," Justin is evidence against it, he was already arguing the writings are "ours," about circumcision/Shabbath/appointed times, and Israelitic identity, long before Constantine.
Does not mean Constantine changed nothing, later imperial involvement drastically changed the relation between theological decisions and government power; separate origin of an idea from later imperial enforcement/standardization. Strongly supported: anti-Yahudi/supersessionary-type arguments and disputes over Shabbath/circumcision/Pasakh/writings/identity predate Nicaea.
Not supported: "Constantine personally invented all of Christianity."
Read it for yourself. Dialogue with Trypho; compare with Constantine's reign and Nicaea 325 CE.
Evidence strength: Directly demonstrable chronology.