The Judaizers were a faction of the early Jewish Christians who regarded the Levitical laws of the Old Testament as still binding on all Christians. They tried to enforce Jewish circumcision upon the Gentile converts to early Christianity and were strenuously opposed and criticized for their behavior by the Apostle Paul, who employed many of his epistles to refute their doctrinal positions. Early Christianity was part of Second Temple Judaism and Gentiles who wished to join the early Christian movement, which was comprised mostly Jewish followers, were expected to convert to Judaism, including circumcision, following the dietary restrictions of kashrut, and keeping the sabbath. During the time period there were also “partial converts”, such as gate proselytes and God-fearers, i.e. Greco-Roman sympathizers which made an allegiance to Judaism but refused to convert and therefore retained their Gentile (non-Jewish) status, hence they were uncircumcised and it wasn’t required for them to follow any of the commandments of the Mosaic Law. The inclusion of Gentiles into early Christianity posed a problem for the Jewish identity of some of the early Christians: the new Gentile converts were not required to be circumcised nor to observe the Mosaic Law. Circumcision in particular was regarded as a token of the membership of the Abrahamic covenant, and the most traditionalist faction of Jewish Christians (i.e., converted Pharisees) insisted that Gentile converts had to be circumcised as well (Acts 15:1). Paul insisted that faith in Christ and His teachings was sufficient for salvation, therefore the Mosaic Law wasn’t binding for the Gentiles. The controversy led to the Council of Jerusalem (Acts Chapter 15), but would continue to persist for the next few decades.
Source: N.T. Wright, What Saint Paul Really Said: Was Paul of Tarsus the Real Founder of Christianity? William B. Eerdmans Publishing Company. June 1997
The Synod of Jerusalem:
The Council of Jerusalem (Acts Chapter 15:1-21)
15 Then certain individuals came down from Judea and were teaching the brothers, “Unless you are circumcised according to the custom of Moses, you cannot be saved.” 2 And after Paul and Barnabas had no small dissension and debate with them, Paul and Barnabas and some of the others were appointed to go up to Jerusalem to discuss this question with the apostles and the elders. 3 So they were sent on their way by the church, and as they passed through both Phoenicia and Samaria, they reported the conversion of the Gentiles, and brought great joy to all the believers.[a] 4 When they came to Jerusalem, they were welcomed by the church and the apostles and the elders, and they reported all that God had done with them. 5 But some believers who belonged to the sect of the Pharisees stood up and said, “It is necessary for them to be circumcised and ordered to keep the law of Moses.”
6 The apostles and the elders met together to consider this matter. 7 After there had been much debate, Peter stood up and said to them, “My brothers,[b] you know that in the early days God made a choice among you, that I should be the one through whom the Gentiles would hear the message of the good news and become believers. 8 And God, who knows the human heart, testified to them by giving them the Holy Spirit, just as he did to us; 9 and in cleansing their hearts by faith he has made no distinction between them and us. 10 Now therefore why are you putting God to the test by placing on the neck of the disciples a yoke that neither our ancestors nor we have been able to bear? 11 On the contrary, we believe that we will be saved through the grace of the Lord Jesus, just as they will.”
12 The whole assembly kept silence, and listened to Barnabas and Paul as they told of all the signs and wonders that God had done through them among the Gentiles. 13 After they finished speaking, James replied, “My brothers,[c] listen to me. 14 Simeon has related how God first looked favorably on the Gentiles, to take from among them a people for his name. 15 This agrees with the words of the prophets, as it is written,
16 ‘After this I will return,
and I will rebuild the dwelling of David, which has fallen;
from its ruins I will rebuild it,
and I will set it up,
17 so that all other peoples may seek the Lord—
even all the Gentiles over whom my name has been called.
Thus says the Lord, who has been making these things 18 known from long ago.’[d]
19 Therefore I have reached the decision that we should not trouble those Gentiles who are turning to God, 20 but we should write to them to abstain only from things polluted by idols and from fornication and from whatever has been strangled[e] and from blood. 21 For in every city, for generations past, Moses has had those who proclaim him, for he has been read aloud every sabbath in the synagogues.”