Establishment of Christianity
Christ Jesus never said to stop being Judaists but be a member of the new religion Christianity.
Accordingly Peter, other Apostles, and even Paul were Judaists. And, the authors of the Gospels are also believed to have been Judaists. However, these Gospels look like criticizing Judaists in general, since Christ Jesus was attacked by Judaist priests of the temple of Jerusalem.
Did Paul, Peter, and the Gospel authors blame only Judaist priests or Judaists in general? Anyway, they seem to have tried to establish true Judaism. And, this true Judaism was open to gentiles, including Romans, because God of Christ Jesus was not only for Judaists but for all the mankind.
However, those Gospels look like having been written to mean that people have to just believe in God and Christ Jesus and there is no need to follow Judaism.
Whether or not they were written with intention to mean that people had to just believe in God and Christ Jesus and there was no need to follow Judaism, the Gospels indicated the direction. So, when the Gospels were written, Christianity was established, leaving the scope of Judaism. But, when Christ Jesus was preaching, Christianity was not established in a strict sense, ironically.
Therefore the core of Christianity is in the Gospels. But there are many mysteries about the Gospels unlike the holy books of Islam, Buddhism, Confucianism, etc.
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Mar 3:14 And he ordained twelve, that they should be with him, and that he might send them forth to preach,
Mar 3:15 And to have power to heal sicknesses, and to cast out devils: