What would have happened if Peter had not defended Paul at the Jerusalem conference?
Had Peter not come to Paul's defense at the Jerusalem conference, Paul's gospel would have died right then and there. Yet God is sovereign and had prepared Peter, even using the house of Cornelius, to defend Paul at that conference twelve years later.
Les explains, regarding Acts 15:12, that after Peter finally came to Paul's rescue, he had always said that if Peter had not done so, Paul's gospel would have died right then and there. But God is sovereign; He had Peter ready. Les maintains the big reason God sent Peter to the house of Cornelius was not only for Cornelius's household, but so Peter would be mentally positioned to come to Paul's defense at the Jerusalem conference 12 years later. God sees the big picture.
Then all the multitude kept silence, and gave audience to Barnabas and Paul, declaring what miracles and wonders God had wrought among the Gentiles by them.
But when Peter was come to Antioch, I withstood him to the face, because he was to be blamed.
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Why was Peter's defense at the Jerusalem council so critical for Paul?
At the Jerusalem conference in Acts 15:12, Peter came to Paul's rescue and defended him; if Peter had not done so, Paul's gospel would have died right then and there.
Why were the Jews at the Jerusalem council convinced by Paul and Barnabas?
If Paul and Barnabas could not have rehearsed miraculous signs and miracles from their ministry, the Jews at the Jerusalem council would never have accepted their report—but since they could, the Jews were convinced.