Acts 15:1

which came down from Judea The words of the new comers would derive authority from this. They would be received as the latest ordinance of the heads of the church at Jerusalem. Thus the mission of enquiry to Jerusalem was rendered necessary.

taught the brethren These were a mixed body, composed of Jews, proselytes and Gentiles (see Acts 11:19-20 and the notes there). Thus it was precisely the place where such a question would arise. Gentile converts who had not passed into Christianity by the gate of Judaism would be sure to be regarded as wanting something, by the people in whose mouths "uncircumcised" had been from old times the bitterest term of reproach. (Cp. 1 Samuel 17:26 and Acts 11:3.) The tense of the verb used implies that these men were persistent in their teaching, they kept constantly to this theme.

after the manner( custom) of Moses The word is found before (Acts 6:14) "the customs which Moses delivered" and signifies those rites and usages which had their foundation in the law (cp. Luke 1:9; Luke 2:42; Acts 21:21) and so were more than a "manner" or "fashion." Cp. also John 7:22, for circumcision as the ordinance given to the people by Moses.

ye cannot be saved A statement likely to cause dissension and questioning among those who had just learnt (Acts 14:27) that "God had opened the door of faith" (independent of the observance of the ceremonial law) "unto the Gentiles."