Today the term “Judaizer” describes the opponents of Paul and Barnabas at the Jerusalem Council (Acts 15 ) and those who sought to preach “another gospel” in the churches of Galatia (Galatians 2:4-12; Galatians 6:12).
The name comes from the the Greek verb Ἰουδαΐ́ζω (Strongs G2450) which means to “judaize” or to “live as the Jews.” In the New Testament, this verb is used only once in Galatians 2:14.
The Judaizers claimed to believe in Jesus as the Jewish Messiah but insisted followers of Jesus cannot be saved unless they also kept the Mosaic law (Acts 15:1).
First-century Jewish nationalists were likely to persecute Jews who were overly friendly to Gentiles. At first, Jewish persecution of Christianity often had more to do with associating with uncircumcised Gentile believers than with the proclamation of Jesus as the Messiah. Forcing Gentile Christians to take on circumcision allowed Jewish believers to prove their loyalty to Judaism and escape suffering. But to do this, Paul reminds his readers, was to deny the sufficiency of the cross (Galatians 3:28).
Dictionaries & Encyclopedias
Liogonier.org: The Judaizers True Motives
Baker’s Evangelical Dictionary of Biblical Theology: Judaizers
McClintock and Strong: Judaizing Christians
The Nuttall Encyclopedia: Judaizers
Wikipedia: Judaizers (be careful with this one; it gets changed)
Got Questions: Who were the Judaizers?
Bible Study Tools: Judaizers
Christianity.com: Why were the Judaizers such big problems in the early church?