Paul was the perfect one to choose being a Jew by nature and yet a Gentile legally as He was a Roman citizen. No one was in comparison to His zeal for God. The Lord counted Paul faithful before his conversion. He was just ignorant of who Jesus Christ really was.
1 Timothy 1
12 And I thank Christ Jesus our Lord, who hath enabled me, for that he counted me faithful, putting me into the ministry;
13 Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief.
14 And the grace of our Lord was exceeding abundant with faith and love which is in Christ Jesus.