Baker's Evangelical Dictionary of Biblical Theology 
Those who adopted Jewish religious practices or sought to influence others to do so. The Greek verb ioudaizo [ Esther 8:17 ) and once in the New Testament ( Galatians 2:14 ). In the Septuagint this verb is used in relation to the Gentiles in Persia who adopted Jewish practices in order to avoid the consequences of Esther's decree ( Esther 8:13 ), which permitted Jews to avenge the wrongs committed against them. The Septuagint not only uses ioudaizo  to translate the Hebrew mityahadim ("to become a Jew"), but adds that these Gentiles were circumcised.
In Galatians 2:14 it means to "live like Jews" (RSV, neb, NASB, Phillips), "follow Jewish customs" ( NIV ), or "live by the Jewish law" (Barclay). The context for this reference is the episode in Antioch when Paul condemns Peter's withdrawal from table fellowship with Gentile Christians. Peter's actions are viewed by Paul as a serious compromise of the gospel of salvation by grace through faith alone, lending support to the position that sought to impose Jewish ceremonial law on the Gentiles. Thus, Paul interprets Peter's withdrawal in terms of its effect in compelling Gentile Christians to live like Jews.
The term "Judaizer" has come to be used in theological parlance to describe 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; 6:12; cf. Philippians 3:2 ). In this sense, "Judaizers" refers to Jewish Christians who sought to induce Gentiles to observe Jewish religious customs: to "judaize." It appears that these individuals agreed with much of the apostolic kerygma but sought to regulate the admission of Gentiles into the covenant people of God through circumcision and the keeping of the ceremonial law. Insisting that "Unless you are circumcised you cannot be saved" ( Acts 15:1 ), these "believers who belonged to the party of the Pharisees" ( Acts 15:5 ) posed a serious threat to the gospel of grace and the uNIVersality of the Christian mission.
Paul's Galatian epistle portrays the Judaizers as having come from the Jerusalem church to his churches in Galatia, stressing the need for Gentiles to be circumcised and keep the law, both for full acceptance by God (legalism) and as the basis for Christian living ( nomism [ Galatians 6:12-13 ). Amidst the rising pressures of Jewish nationalism in Palestine during the mid-first century, and increased Zealot animosity against any Jew who had Gentile sympathies, it would appear that these Jewish Christians embarked on a judaizing mission among Paul's converts in order to prevent Zealot persecution of the Palestinian church.
R. David Rightmire
Bibliography . F. F. Bruce, Galatians ; J. Dunn, Unity and Diversity in the New Testament ; R. Fung, Galatians ; W. Gutbrod, TDNT, 3:383; R. Jewett, New Testament Studies ; R. Longenecker, Galatians .
Webster's Dictionary 
(n. pl.) See Raskolnik.
The Nuttall Encyclopedia 
A party, called also
n the primitive Church who sought to overlay the simple ordinances of Christianity with Judaic observances and rites, "a yoke which neither they nor their fathers were able to bear."