20 I assure you before God that what I am writing to you is no lie. -Galatians 1:20 BSB
Paul was assuring the saints in Galatia that everything he has said to this point and everything else included in this letter is nothing but the truth and can be trusted. Regarding his revelations, biblical understanding (see verse 11 and its commentary), and his relationship with the Lord Jesus Christ.
19 But I saw none of the other apostles except James, the Lord’s brother. -Galatians 1:19 BSB
The most likely reason why Paul decided to visit James was that he was the leader of the church in Jerusalem, see Acts 21:17-18.
In the time of Jesus’ ministry, His brothers, including James, didn’t believe that He was the Messiah, and we can also include, even though the Scripture doesn’t say, the virgin birth of Jesus Christ if their mother revealed that to them. The reason being is that they didn’t believe that their brother performed works by the finger of God (Luke 11:20). The works done by Jesus seem to indicate that He was extraordinary, making Him the Son of God, see John 10:38, which was something that the brothers didn’t want to believe. See below.
3 So Jesus’ brothers said to Him, “Leave here and go to Judea, so that Your disciples there may see the works You are doing. 4 For no one who wants to be known publicly acts in secret. Since You are doing these things, show Yourself to the world.” 5 For even His own brothers did not believe in Him. -John 7:3-5 BSB
It wasn’t after the resurrection of Jesus Christ when He appeared to James that he accepted Jesus as his Lord and Savior, see 1 Corinthians 1-7.
James became one of the pillars of the faith, an elder of the church in Jerusalem, and was used greatly by the Lord to influence the Jews that Jesus indeed was the promised Messiah.
18 Only after three years did I go up to Jerusalem to confer with Cephas, and I stayed with him fifteen days. -Galatians 1:18 BSB
After three years of Paul’s conversion to Christianity—having an encounter with Jesus Christ, after His resurrection and ascension to heaven, see Acts 9:1-6, he visited Peter.
Paul stayed with Peter for several days, as he was one of the pillars of the faith, see Galatians 2:9, most likely getting more insights about Jesus and what He taught while He was on earth with His disciples so that Paul would be much more informed and equipped for preaching the gospel.