The Apostle Paul had been explaining that after he encountered the risen Jesus, he had no contact with other apostles, churches or believers for three years (Galatians 1:15-17).  During that time in Arabia the Lord taught Paul the meaning of the Scriptures - when they were read as intended, they looked forward to the person and work of Jesus Christ.  

Peter ('Cephas' in Greek) and James were still alive when this letter was written and could validate all that Paul said [James was the half-brother of the Lord Jesus and writer of the Letter of James (Mark 6:3) – note that James the apostle had already been executed (Acts 12:1-2)].  When they eventually met, their understanding of the gospel was identical to Paul’s.  So his new understanding came from the Scriptures and from Jesus Christ: not from human wisdom or even other apostles.
 
Paul had been transformed.  He had met the living Jesus and repented of his former life.  Instead of destroying the churches, he now did all he could to build them up.  His method was simple: to preach the gospel and to reveal Jesus as the Son of God as well as the Man of Sorrows, and then how to live to please Him (1 Corinthians 1:17-25).  Paul’s radical personal change excited praise to God among many who had never met him.
 
When God changes a person’s heart, He also changes their behaviour.  Your life should also have some of the marks of divine transformation, and your character acquire the hall-marks of the Holy Spirit’s presence (Galatians 5:22-23).  It is right that people should rejoice when your behaviour demonstrates that your heart has changed.  That is why it is important to live with spiritual integrity – be what you are.  And be ready with an answer to explain the reason for your new way of life (1 Peter 3:15-16).  If another Christian cannot look at you and thank God for His grace, something is wrong!  Every internal transformation done by God will have evidence.  It is also important to thank God for others: instead of criticising the parts of their behaviour you do not like, it is better to thank the Lord for whatever is good and brings glory to Him … and leave Him to deal with the parts of their lives which still need transformation (Philippians 1:6; Philippians 2:13).