In theory, Judaism has always welcomed any person who wished to be received into that religious community on the condition of circumcision and the desire to follow the Law of Moses. But in practice the convert to Judaism, or proselyte, seldom obtained the full acceptance that the Scriptures and rabbinic teachings stated he should. The reason for this probably lies in the suffering that outsiders inflicted on the community throughout the history of Israel. Thus, the people were careful not to compromise survival with an "open door" policy on conversion.
However, during the first century a large number of Gentiles were received into the Jewish community as full members. They agreed to keep the Law according to the traditions of the elders, to offer ritual sacrifices in the temple, and to attend the synagogue regularly. Once the Jewish authorities were satisfied that a man was sincere, he was instructed in the Law and history of the Jewish people, then he was circumcised. After his wound healed, he was stripped of all his clothes and led by three witnesses, who had sponsored him, to a tank or pool to be baptized. His sponsors repeated the commandments of the Law, which he vowed to keep, and he was plunged under the water. He then made his first offering as a Jew.
There were also many Gentile men who wished to associate themselves with the Jewish community without becoming full members. These men worshiped and studied in the synagogue but were not circumcised. They kept a portion of Jewish law, without vowing to keep the whole complex set of traditions. These "semiproselytes" were known as "God-fearers," and included such people perhaps as the centurion who contributed funds to build the Capernaum synagogue.
It is probable that former Gentiles who converted to Judaism became more committed and more intense than those raised in the faith. It is this tendency of converts to follow their new faith excessively that led Jesus to tell some Pharisees that their converts tended to become even more Pharisaic than they were themselves (Pharisees energetically sought new converts). These zealous converts became twice as deserving of punishment in the afterlife than even the Pharisees who had converted them (Matt. 23.15)