Who was foreordained Son of God by an act of power in keeping with the holiness of his spirit, by resurrection from the dead, Jesus Christ, our Lord, through whom we have received the grace of apostleship to bring about obedience to faith among all the nations for his name's sake.
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Notice that St. Paul explains that Jesus Christ is Son of God "by an act of power in keeping with the holiness of his spirit," and thus ensures that there is no thought of Jesus being Son of God in the way that pagans believe in biological and actual familial relationships among individual "god" personages. St. Paul explains through his phrasing that Jesus Christ is the Son of God because of the holiness of spirit (that was given to Jesus directly by God through the Holy Spirit) and his resurrection from the dead, which demonstrates his God given power over life and death, specifically returning to life after death.
Notice also the reminder that being an apostle is a gift of grace, but the duty is to bring about obedience to faith to God. In other words, it is not just a lofty title of "enlightenment," but a duty to bring the faithful to the worship of the one true God in the name of Jesus Christ.