Galatians 5:11

Evidently some people were saying Paul advocated circumcision. He may have preached it before his Damascus road conversion, but since then he had stopped. Probably Paul meant that the accusation of his critics that he preached circumcision when it suited him was not true (cf. 1 Corinthians 7:18). [Note: Boice, p. 490.] Paul thought it wise for some Christians, such as Timothy, to undergo circumcision for the sake of effective ministry (Acts 16:3). However, he did not teach that it was necessary for salvation.

Paul’s point here was that if he taught circumcision was necessary for salvation the Judaizers would not have persecuted him. If people need circumcision, they do not need the cross of Christ. The legalists opposed Paul’s preaching of the Cross because it implied that people are unable to please God themselves.

"The skandalon [stumbling block] of the cross, for Jews (cf. 1 Corinthians 1:23), lay in the curse which it involved for one who was hanged on it (cf. Galatians 3:13). That one who died such a death should be proclaimed as Lord and Christ was intolerable. In the eyes of Gentiles the idea that salvation depended on one who had neither the wit nor the power to save himself from so disreputable a death was the height of folly. But there is a more general skandalon attached to the cross, one of which Paul is probably thinking here: it cuts the ground from under every thought of personal achievement or merit where God’s salvation is in view. To be shut up to receiving salvation from the crucified one, if it is to be received at all, is an affront to all notions of proper self-pride and self-help-and for many people this remains a major stumbling-block in the gospel of Christ crucified. If I myself can make some small contribution, something even so small as the acceptance of circumcision, then my self-esteem is uninjured." [Note: Bruce, pp. 237-38.]

In short, Paul’s gospel was a stumbling block for two reasons: it presented a crucified Messiah and it advocated a way of salvation apart from circumcision and the Law.