Galatians 2:14

"When I saw that they were not acting in line with the truth of the gospel, I said to Peter in front of them all, 'You are a Jew, yet you live like a Gentile and not like a Jew.  How is it, then, that you force Gentiles to follow Jewish customs?'" (NIV)

Today was the perfect day to take notes on this verse because I heard something yesterday that explains this really well.  Peter, who was born a Jew and had always lived by Jewish practices, was acting in such a way that implied that Gentiles had to obey Jewish customs in order to be saved.  But Peter knew that Jesus was the only way to heaven, and he "[lived] like a Gentile," meaning he lived according to the message that salvation comes from "Jesus plus nothing."  Having already been circumcised before coming to faith, Peter had nothing left in the Law that he was required to obey.  It was Paul's reasoning, then, (and Peter's original reasoning) that not even circumcision was required.  Yet Peter had turned to join the Judaizers in requiring circumcision from the uncircumcised.  In other words, one had to be a Jew by the Law in order to be Jewish by faith.  Paul pointed out this flaw in the Judaizers' "gospel" to Peter publicly.  This was the same false message that had come to the Galatians, so Paul used the same argument to bring the Galatians back to the truth that Jesus saves and nothing else is required but faith.

Galatians 2:13

"The other Jews joined him in his hypocrisy, so that by their hypocrisy even Barnabas was led astray." (NIV)

I see a chain of consequences here.  When the men from James came, Peter became afraid of creating conflict, so he fell back on his old ways as a Jew without Christ.  His fear led to hypocrisy, and the masses saw and imitated his example as a leader.  Then Barnabas, a strong Christian, was "led astray" because of the masses, who were following their leader.  Paul saw the consequences of this path of digression away from the gospel, and he knew he had to put an end to the hypocrisy immediately, particularly Peter's hypocrisy.  Once the leader's faith is reestablished, the masses (including Barnabas) would follow his example.