Why was St. Paul never declared a Doctor of the Church?

Why was St. Paul never declared a Doctor of the Church? In many ways, he appears to be the first Doctor of the Church. His writings are not only doctrinal but scriptural. Is it because he’s an Apostle? Can one not be a doctor and an Apostle?

St. Paul is an apostle, a martyr, and an author of Sacred Scripture. There is no honor higher than those things.

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