Romans 1:1

In this paragraph we have, I. The person who writes the epistle described (Rom 1:1): Paul, a servant of Jesus Christ; this is his title of honour, which he glories in, not as the Jewish teachers, Rabbi, Rabbi; but a servant, a more immediate attendant, a steward in the house. Called to be an apostle. Some think he alludes to his old name Saul, which signifies one called for, or enquired after: Christ sought him to make an apostle of him, Act 9:15. He here builds his authority upon his call; he did not run without sending, as the false apostles did; klētos apostolos - called an apostle, as if this were the name he would be called by, though he acknowledged himself not meet to be called so, Co1 15:9. Separated to the gospel of God. The Pharisees had their name from separation, because they separated themselves to the study of the law, and might be called aphōrismenoi eis ton nomon; such a one Paul had formerly been; but now he had changed his studies, was aphōrismenos eis to Euangelion, a gospel Pharisee, separated by the counsel of God (Gal 1:15), separated from his mother's womb, by an immediate direction of the Spirit, and a regular ordination according to that direction (Act 13:2, Act 13:3), by a dedication of himself to this work.

— Matthew Henry's Concise Commentary (public domain)