a. Jewish Christians, possibly from Jerusalem, possibly of an even stricter stripe (Hays advocates a version of this position in the annotations in the HarperCollins Study Bible. Sheila Briggs in The New Oxford Annotated suggests that we simply do not know who Paul’s opponents were exactly, p. 310).

For:

1:6-2:21 recounts differences between Paul’s attitudes toward Gentiles and those of various other Jewish Christians including James and Peter. The others insist on circumcision and keeping of certain food laws.

Paul’s lengthy interpretation of Abraham in 3:6-18 and 4:21-5:1 likely rebuts their citation of passages from Genesis 15 and 17.

Jewish Christians might have allowed the Galatian Gentiles some freedom from Jewish Torah so long as they were circumcised, i.e., they may not have required them to keep the whole law.

Against:

Although Paul argues for his independence from the Jerusalem leaders, he does report their support for his gospel.

The opponents claim that Paul depended on the Jerusalem church for his gospel, which wouldn’t make sense if they were from the Jerusalem church.

Paul’s description of the opponents as "those who are being circumcised" (a reading in some ancient manuscripts of 6:13 instead of "the circumcised") makes no sense if the opponents are Jewish Christians.

Similarly, Jewish Christians would have known that circumcision required one to keep the whole law (5:3).

The opponents as Jewish Christians doesn’t explain the worship of the elemental spirits (4:9) and their tendency towards libertinism--excessive freedom or ethical license (5:13).

b. Gentile Judaizers, possibly from within the congregation itself (supported by J. Munck and L. Gaston)

For:

Paul’s description of the opponents as "those who are being circumcised" in 6:13 and their ignorance of the requirement of keeping the whole law if one is circumcised (5:3) make more sense if these are Gentiles who study the Torah and decide that circumcision is necessary for their salvation. For example, read Genesis 15 and Genesis 17:1-14.

"Lloyd Gaston has shown, that oddly enough, there were Gentile Judaizers in Asia Minor [ urging the adoption of circumcision and other Jewish practices] although in a somewhat later period ." (Roetzel, 99). Ignatius’ Letter to the Philadelphians says in 6:1: "If anyone interpret Judaism to you do not listen to him; for it is better to hear Christianity from the circumcised [such as Paul] than Judaism from the uncircumcised [Gentile Judaizers]" (cited in Roetzel, 99).

Paul does not call his opponents "outsiders" in the letter.

Against:

Like Option a, this option doesn’t explain the worship of elemental spirits and libertinism.

c. Hellenistic Syncretists/Radicals (supported by Betz and Schmithals who sees them as some sort of gnostics)

This one incorporates some of the argument of Option B and adds the notion of syncretism of Jewish and native folk practices. This would explain the worship of elemental spirits. Circumcision might have been viewed as something to ward off cosmic entities (see Barr, p. 101), thus freeing the Galatians from the spirits’ power over them.