In view of the gospel being available to the Gentiles, Paul became an apostle to them. It was because of his preaching to the Gentiles that events involving the Jews in Jerusalem caused him to be in a Roman prison when he wrote the Ephesian letter.
Paul digressed from his main thought and described himself and his apostleship. He reminded the Ephesians that he had received the gospel by revelation and not from ordinary men. His commission was to preach that gospel to the Gentiles.
As Paul wrote of his apostleship, he described the mystery of Christ as it related to the Gentiles. He explained that this mystery was their acceptance into the fellowship of Christ as equals with the Jews. Even though it had been prophesied in the Old Testament, it was not revealed until Peter had received it from God prior to the conversion of Cornelius and his household.
Paul looked upon himself as least of all the saints because of his history of persecuting the church. Nevertheless, he accepted the commission to preach to the Gentiles and asked for their support during his tribulations as their apostle.