Let’s read Galatians 1:18-20 (NLT)
18 Then three years later I went to Jerusalem to get to know Peter, and I stayed with him for fifteen days. 19 The only other apostle I met at that time was James, the Lord’s brother. 20 I declare before God that what I am writing to you is not a lie.
Verse 18
During those years, he was trained by Jesus Himself. With this kind of training, Paul truly became qualified to be called an apostle and provide a trustworthy message.
After his conversion and being trained by Jesus, Paul headed to Jerusalem at last, where he consulted with the apostle Peter (whose nickname was Cephas). Paul was only there for 15 days though, because he had to run for his life after preaching about Jesus (Acts 9:29).
Verse 19
In addition to learning from Peter, Paul met with James, Jesus’ brother. James was serving as a leader in the Christian church of Jerusalem at the time. His book (James) compliments Paul’s teachings on salvation by grace alone quite beautifully.
Verse 20
Since the Judaizers were accusing Paul of being a liar and a heretic, he takes a moment to interject his story here. His saying, “I declare before God” is basically Paul saying, “As God is my witness, I’m not lying to you!” He wanted the Galatians to understand that he was truly sent out by Jesus and was sharing God’s truth.