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.