“Set apart” might refer to Paul’s separation from Judaism, or his missionary activity (Acts 13:2.) But it is more likely that the separation that Paul has in mind is his “consecration to a future task,” specifically the task of bringing the Gospel to the Gentiles. Paul uses the word ἀφορίζω (aforizo) in both Gal 1:15 and this passage to describe his calling. But the word is used in the Septuagint for setting apart a firstborn man or animal to God (Numbers 15:20) or consecrating Levites to God’s service (Numbers 8:11). In a few cases, the word is used to describe God separating Israel from the nations to be his people (Lev. 20:26).
Paul chose this word to emphasize his belief that his life was something of a sacrifice to the Lord. A sacrifice no longer lives its own life, but he is wholly given over to the purpose for which he was chosen. But Paul takes this a bit further than a literal sacrifice – Paul set apart by God from birth (Gal 1:15)! This is more like the prophet Jeremiah, who says that he was set apart for prophetic office before he was born (Jer 1:5).
Paul is separated for the purpose of the gospel of God. Since most Christians have an idea of what the “gospel” is they overlook the rich background behind this word. There is both an Jewish and Greco-Roman background to the “good news.”
Looking at the “good news” from the Jewish perspective, the word is associated with the coming time of eschatological judgment and salvation. In the Hebrew Bible, the word בשׂר (bashar) is used in Isa 61:1, for example, to describe the activity of the anointed one, and is associated with both the coming of God’s salvation and his vengeance. This word is translated as εὐαγγέλιον (euangelion) in the LXX. Jesus used the same phrase to describe both his teaching and healing ministry in Galilee (Matt 11:5 and Luke 7:22).
But the Roman world used the phrase “good news” as well. They considered important events in the life of the Emperor as “good news” and celebrated them. It is possible that this word was chosen intentionally to contrast the real good news of salvation with the false peace which comes from the Roman “good news.” This is a counter-cultural and dangerous idea, since it says that the “good news” about the emperor fades into insignificance in the light of the Gospel of God.
On one level, Paul is unique as one who was set apart by God to be the light to the Gentiles. But on another level, Paul is a model for all Christians. Later in the letter Paul says that all believers are to be a “living sacrifice” (12:1). If Christians really lived out their “set apart” calling, I think that the gospel could again be just as counter-cultural as it was in A.D. 55.