Constantine and the Council of Nicaea decided which books went into the Bible

This claim gets repeated constantly: The Da Vinci Code popularized it, and it shows up in documentaries claiming that if the Gospel of Judas or Mary had "won," Christianity would look different today. The actual history runs the other way. By the time of the Council of Nicaea in 325 AD, the canon conversation was almost entirely over. The records of Nicaea show both sides of its actual dispute (Arius and the orthodox bishops arguing over whether the Son was of the same substance as the Father), arguing from the same set of scriptures. Nobody at Nicaea is citing Thomas or Peter or debating whether to include them. Constantine also didn't convert the empire to Christianity; that was Theodosius, decades later.

What actually happened with the New Testament's 27 books was recognition, not creation, using three criteria: apostolicity (does it trace to an apostle or someone close to one), orthodoxy, and catholicity (was it accepted across the wider church). Apostolicity was the gatekeeper. Two books that were popular and doctrinally orthodox (the Shepherd of Hermas and the Epistle of Barnabas) still didn't make the cut, because neither could be reliably tied to an eyewitness of Jesus; being loved and being orthodox wasn't enough. Meanwhile writers like Irenaeus in the second century were already saying it's "not possible" for there to be more or fewer than four Gospels, naming Matthew, Mark, Luke, and John specifically, and the Muratorian Fragment, an early canon list, recognizes 22 of the 27 books with no debate. The theologian F.F. Bruce put it simply: the church didn't make these books authoritative by listing them: it listed them because it already recognized their authority. The famous "lost gospels" themselves, like the Gospel of Thomas found in the Nag Hammadi library in 1945, are second-century-or-later texts written under borrowed names, long after any of the apostles were alive, and they simply never had the widespread, early acceptance the four Gospels already had a century before Nicaea convened.