When and hw did Christianity become a religion as opposed to a Jewish sect? Jesus and his followers were all Jews.
Gradually, in a process that began around 50 CE and was generally complete by the start of the 4th century CE.
Jewish Christians in the 1st and 2nd centuries still worshipped in synagogues and still held to Jewish holy days, identified as ethnically Jewish, and engaged with the Jewish intellectual tradition involving both written and oral Torah. Sects that completely rejected the Jewish heritage of Christianity (i.e. Marcionism) were not mainstream within the early Christian movement, and were met with scorn even among gentile converts. But the adherence to Jewish law became less and less, accelerated by the Jewish-Roman Wars. Gentile Christianity overtook it, along with an emerging Church hierarchy.
The Council of Nicaea
did condemn the Jewish-Christian Ebionites as a heresy, but that was more of a final-nail-in-the-coffin thing, as the sect had almost disappeared by that point in time. Honestly, the Council of Nicaea gets a
lot of stuff assumed about it that just didn't happen; the main thing it did was set down how to figure out the date of Easter, and decide on Christology against Arianism. It did
not determine which books were in the Bible-- that had happened through consensus over the previous couple centuries, and the council that
finalized it was like 70 years
after Nicaea. And really, Nicaea didn't actually solve most of the issues it was convened to resolve, since they had to hold another council and a half-dozen synods over the next century, and Arian Christianity remained a major sect until the 600s.
In any case, the Jewish Diaspora was a bigger part of the schism between Christianity and Judaism than is usually considered, in my opinion. Prior to that, oral and written Jewish law was interpreted by both the Sanhedrin and by Pharisaic sages, and rituals were conducted at a central Temple. But between the destruction of Jerusalem in 70 CE and the last Jewish-Roman War in 135 CE, these institutions were destroyed, and the Jewish population was scattered throughout the Roman world and further east into Persia. Christians who were still connected to Jewish practices and scholarship were, like other Jews, suddenly bereft of a legal, religious, and cultural touchstone for that part of their traditions. So Jews that were Christian drifted in a direction similar to their Gentile Christian co-religionists, because it kinda became all they had left.
The other Jewish communities ultimately came together under Rabbinic Judaism, wrote down Oral Torah, and developed the Talmud as a way to survive as a distinct ethnic community, but it was a very long and difficult road. And the course it took was just as much a reaction to the perception that Jewish Christians were heretical, and consciously evolving in a different direction.