What does Acts 15:17 mean?

Despite Jesus' strong words against them (Matthew 23:15), not all Pharisees were hardened against His teaching (John 3:1; 7:50–51). After His ascension, many Pharisees accepted Christ as savior and joined the church. Unfortunately, they brought their strong legalism with them. Pharisee-Christians have been harassing Gentile believers in Syrian Antioch, telling them they must be circumcised and follow the Mosaic law to be saved by the Jewish Messiah. However, the church in Antioch is home to Paul and Barnabas who not only vehemently disagree, they take the matter to Jerusalem. There, they confer with the apostles and elders of the church, to decide the matter once and for all. Peter has already reminded the council of his interaction with Cornelius, wherein Peter watched the Holy Spirit fall on a houseful of Gentiles with no circumcision, baptism, or even laying on of hands (Acts 15:1–11).

Now James, Jesus' half-brother and the pastor of the church in Jerusalem, quotes "the prophets" (Acts 15:15). He reminds the council that God had always planned to accept Gentiles—not just Gentiles who became proselytes to Judaism—into His kingdom. Starting in Acts 15:16, James quotes Amos 9:11–12 with insertions from a couple of other prophets. In Acts 15:16, he explained that God promised to raise up the nation of Israel, now occupied by Romans, so that David's kingdom would once again stand. Now, he reminds the council that Amos' prophecy promises Gentiles will be included with those who worship God.

James quotes Amos 9:12 from the Septuagint, which reads:

…that the remnant of men,
and all the Gentiles,
upon whom my name has been called,may earnestly seek,
saith the Lord who does all these things.

The first line is markedly different from our versions of Amos 9:12, which starts, "that they may possess the remnant of Edom." Edom was the nation conquered and ruled by Jacob's brother Esau. God cursed them with destruction in part because they continually harassed Israel but finally when they turned over Jews who were fleeing from Nebuchadnezzar and the Babylonian army (Obadiah 1:10–14). Herod the Great's father and—obviously—his sons and grandsons were Edomites, then called Idumeans. The nation as an entity was destroyed by the Romans the same time as Israel. Unlike Israel, however, God left Edom no remnant.

The ESV version of Amos 9:12 continues, "and all the nations who are called by my name." In this text, it appears God is promising Israel they will rule over Edom and all the other nations. That's true in that Jesus will reign over Gentiles and the Gentile nations will serve Israel. The Septuagint emphasizes the fact that God planned, all along, to call Gentiles to bear His name, meaning they will recognize Him as their sovereign authority and Jesus as King. The alteration from "Edom" to "nations" is consistent with Hebrew literature; "Edom" is a synecdoche—a figure of speech wherein a part is named to represent a whole—which represents Gentile nations in general. We see this in several places in the Old Testament where the tribe of Ephraim is used to mean the entire northern kingdom of Israel such as Isaiah 7:2.

The last line of Amos 9:12 in the Septuagint reads, "says the Lord who does all these things," which is basically the same as the ESV. James replaces, "who does these things" with Isaiah 45:21: "who declared it of old." The Septuagint emphasizes God had intended these things from the start: "who made from the beginning these things that are to be heard." James is using gezerah shavah. This is a Jewish literary practice melding parts of different sources that have a similar message, to add depth to the argument.

James drops off the part about God doing these things from the Septuagint version of Amos 9; God is consistent in thought, word, and deed, and for Him to say He will do something means He will do it. Instead, James finishes with the point he wants to make about the topic at hand. While Amos records that God will make these things happen, James uses Isaiah's quote about how God makes these things known (Acts 15:18). Is he misquoting the Hebrew Scriptures? No, James is showing how God's Word as promised by "the prophets" (Acts 15:15) is being fulfilled right in front of them.