Romans 1:1

Paul, a servant of Jesus Christ
The name of the author of this epistle is Paul, who formerly was called Saul. Some think his name was changed upon his own conversion; others, upon the conversion of the Roman deputy Sergius Paulus, ( Acts 13:7 Acts 13:9 ) ; others, that he was so called from the littleness of his stature; but rather it should seem that he had two names, which was usual with the Jews; one by which they went among the Gentiles, and another by they were called in their own land; (See Gill on Acts 13:9). "A servant of Jesus Christ"; not a servant of sin, nor of Satan, nor of man, nor of Moses and his law, nor of the traditions of the elders, but of Jesus Christ; and not by creation only, but by redemption, and by powerful efficacious grace in conversion; which is no ways contrary to true liberty; nor a disgraceful, but a most honourable character; and which chiefly regards him as a minister of the Gospel:

called to be an apostle:
an apostle was one that was immediately sent by Christ, and had his authority and doctrine directly from him, and had a power of working miracles from him, in confirmation of the truth of his mission, authority, and doctrine; all which were to be found in the author of this epistle, who did not thrust himself into this office, or take this honour to himself, of which he always judged himself unworthy, but was "called" to it according to the will, and by the grace of God:

separated unto the Gospel of God.
This may regard either God's eternal purpose concerning him, his preordination of him from eternity to be a preacher of the Gospel, to which he was separated from his mother's womb, ( Galatians 1:15 ) ; or the separation of him to that work made by the order of the Spirit of God, ( Acts 13:2 ) . The phrase used is either in allusion to the priests and Levites, who were separated from their brethren the children of Israel, to their sacred employments; or rather to the apostle's having been (vwrp) , "a Pharisee", which signifies "one separated", as he was now; only with this difference, before he was separated to the law, but now "to the Gospel", to preach and defend it, which he did with all faithfulness and integrity; the excellency of which Gospel is signified by its being called "the Gospel of God": he is the author of it; his grace is the subject of it; and he it is who commits it to men, qualifies them for the preaching of it, and succeeds them in it.