Through whom we have received grace and apostleship to bring about the obedience of faith among all the Gentiles for His name’s sake, among whom you also are the called of Jesus Christ.
Calvinists can’t use the “Well, it just means ‘groups’” defense, since this refers to “all the Gentiles.”
John Calvin comments: “It was not enough for Paul to have been appointed an apostle, unless his ministry had reference to the making of disciples. He therefore adds that his apostleship extends to all the Gentiles.” (Calvin’s New Testament Commentaries: Romans and Thessalonians, p.18, emphasis mine)
Regarding “all the Gentiles,” the New Living Translation paraphrases: “Through Christ, God has given us the privilege and authority to tell Gentiles everywhere what God has done for them, so that they will believe and obey him, bringing glory to his name.” (emphasis mine) And what did He do for them? Calvary is what He did for them. This means that Jesus died for everyone, and all who partner in the ministry are “the called of Jesus Christ” in order to reach these people. But if Limited Atonement is true, then God hasn’t “done” anything for them, at least in a salvific sense, or an eternally beneficially sense, and so, what good news do you really have for any random Gentile, if not every Gentile is so loved by God? After all, if you cannot assure them that Jesus loves them, died for them, and rose again on their behalf, then you are merely offering them a could-be, might-be, hope that you are secretly elect in an eternal raffle, find out in the end, hope for the best, Gospel. I think that the biblical Gospel offers more hope than a presumption.