
The Lord Jesus chose 12 men (and trained them) to take the Gospel to the people of Israel, the Jews. Before His death He was betrayed by one of them, Judas, who later committed suicide. After the Lord’s Resurrection and Ascension, the eleven knew there had to be 12 apostles, so they elected one, Matthias, to replace Judas. These men were to take the message to all the Jews throughout the world.
So, why Paul, (who was Saul, who persecuted believing Jews)?
Was Paul supposed to be the 12th apostle? NAY! He was a Jew, chosen by God to take the message of salvation to the gentiles.
In His letters to Believers (Gentiles) in Corinth he writes:
1 Corinthians 1:1 Paul called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother,
2 Corinthians 1:1 Paul, an apostle of Jesus Christ by the will of God, and Timothy our brother, unto the church of God which is at Corinth, with all the saints which are in all Achaia:
In His letter to Believers (Gentiles) in Ephesus he writes:
Ephesians 1:1 Paul, an apostleof Jesus Christ by the will of God, to the saints which are at Ephesus, and to the faithful in Christ Jesus:
In His letter to Believers (Gentiles) in Colossi he writes:
Colossians 1:1 Paul, an apostle of Jesus Christ by the will of God, and Timotheus our brother,
In His letters to Timothy he writes:
1 Timothy 1:1 Paul, an apostle of Jesus Christ by the commandment of God our Saviour, and Lord Jesus Christ, which is our hope;
2 Timothy 1:1 Paul, an apostle of Jesus Christ by the will of God, according to the promise of life which is in Christ Jesus,
And to Titus he writes:
Titus 1:1 Paul, a servant of God, and an apostleof Jesus Christ, according to the faith of God’s elect, and the acknowledging of the truth which is after godliness;
To those in Rome, he is more specific:
Romans 11:13 For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office:

Why Paul? He was the best man for the Job!
Just read about Him after his calling. Acts 9, Acts 22.
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