But when Peter came to Antioch, I had to oppose him to his face, for what he did was very wrong.12 When he first arrived, he ate with the Gentile Christians, who were not circumcised. But afterward, when some friends of James came, Peter wouldn't eat with the Gentiles anymore. He was afraid of criticism from these people who insisted on the necessity of circumcision.13 As a result, other Jewish Christians followed Peter's hypocrisy, and even Barnabas was led astray by their hypocrisy.
Peter is altering his actions because of his predisposition to justify his faith through his actions. He feels that with whom he associates, how he dresses, what he consumes – circumcision, inform or influence his salvation.
Why does Paul “…oppose him to his face”? Paul understands that Peter’s hypocrisy needs to be sharply denounced because it is, in fact, contrary to the very core of the gospel message. If we can justify, save or even just improve ourselves by changing the way we act then Christ’s death was a waste. The whole idea is that Jesus came for us while we were still sinners, so then when we think that he loves us more when we act better, we basically say, “No thanks, no need” to Jesus.
10 For I want you to understand what really matters, so that you may live pure and blameless lives until the day of Christ's return.11 May you always be filled with the fruit of your salvation—the righteous character produced in your life by Jesus Christs—for this will bring much glory and praise to God.
If the “Fruit of our salvation” is Christ’s work within us, does that mean that our choices don’t matter? Of course not. But our actions are to be in response to his work in our hearts through the Holy Spirit. I act in accordance with scripture because Jesus has impacted my desires - I respond to his grace. Conversely, I will never change my own heart by acting in accordance with scripture. I can’t. I, when I try to act holy to be more sanctified, actually become MORE of a hypocrite. But when my perspective is one of humility and contriteness of heart, “I will change for Jesus because he changed me”, I am accepting the reality that without him I am a hypocrite. Hypocrisy is not a series of actions that can be lessened with effort, it is a state to be saved from. In order to not be a hypocrite I have to admit that I am one.