What did the men from Judea teach the Gentile believers in Antioch?
The men from Judea taught the Gentile believers at Antioch that unless they were circumcised after the manner of Moses, they could not be saved. They were Judaizers still under the law, insisting that salvation required bringing the Gentiles under Israel's law.
Les teaches that Acts 15 is a direct parallel to Galatians 2, describing the same set of events. Certain men came down from Judea and taught the brethren, the Gentile believers at Antioch, that except they be circumcised after the manner of Moses, they cannot be saved. This occurred about 52 AD, roughly 22 or 23 years after Pentecost. These Jewish believers from Jerusalem were still so saturated with the law that they tried to bring Paul's Gentile believers at Antioch under it.
And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved.