The first believers in Yeshua were a Jewish sect known as “Nazarenes” or in Hebrew “Netzarim” (Acts 24:5).
The “church father” Jerome (4th Cent.) described these Nazarenes as those “…who accept Messiah in such a way that they do not cease to observe the old Law.” (Jerome; On. Is. 8:14).
Elsewhere he writes:
Today there still exists among the Jews in all the synagogues of the East a heresy which is called that of the Minæans , and which is still condemned by the Pharisees; [its followers] are ordinarily called ‘Nazarenes’; they believe that Messiah, the son of God, was born of the Virgin Miriam, and they hold him to be the one who suffered under Pontius Pilate and ascended to heaven, and in whom we also believe.”
(Jerome; Letter 75 Jerome to Augustine)
The fourth century “church father” Epiphanius gives a more detailed description:
But these sectarians… did not call themselves Christians–but “Nazarenes,” … However they are simply complete Jews. They use not only the New Testament but the Old Testament as well, as the Jews do… They have no different ideas, but confess everything exactly as the Law proclaims it and in the Jewish fashion– except for their belief in Messiah, if you please! For they acknowledge both the resurrection of the dead and the divine creation of all things, and declare that G-d is one, and that his son is Yeshua the Messiah. They are trained to a nicety in Hebrew. For among them the entire Law, the Prophets, and the… Writings… are read in Hebrew, as they surely are by the Jews. They are different from the Jews, and different from Christians, only in the following. They disagree with Jews because they have come to faith in Messiah; but since they are still fettered by the Law–circumcision, the Sabbath, and the rest– they are not in accord with Christians…. they are nothing but Jews…. They have the Goodnews according to Matthew in its entirety in Hebrew. For it is clear that they still preserve this, in the Hebrew alphabet, as it was originally written. (Epiphanius; Panarion 29).