Been following this discussion at a distance;
The main difficulty concerns the interpretation of the quote from Amos. Most premillennial commentators interpret it to refer to the second coming of Christ and the future restoration of David’s throne, followed by worldwide witness to the Gentiles in the millennium. Thus James would be arguing that since Amos predicted the future inclusion of the Gentiles in God’s purpose apart from their becoming Jewish proselytes, there is therefore no need for them to become Jewish proselytes in the present situation (John MacArthur, Jr., Acts 13-28 in The MacArthur New Testament Commentary [Moody Press], p. 69).
It seems to me, however, that James is using the quote from Amos to refer to what God was doing in the present, not to what He would do in the future. This is not to deny a future aspect and greater fulfillment of the prophecy in the millennial kingdom. But I think James uses the quote to establish that God’s purpose in the present age includes the calling of the Gentiles apart from their becoming Jews (see Stedman's sermon
Acts 15:1-21 What's God Doing?).
Here are some premill guys, but if you reverse it, it supports the other view;
Warren Wiersbe assesses this difficult passage quoted by James as follows - Note that (James) did not state that what Peter, Paul, and Barnabas had said was a
fulfillment of this prophecy. He said that what Amos wrote
agreed with their testimony. A careful reading of
Amos 9:8-15 reveals that the prophet is describing events in the end times, when God will regather His people Israel to their land and bless them abundantly. If we "spiritualize" these promises, we rob them of their plain meaning and James' argument falls apart. Amos also prophesied that the fallen house ("tent") of David would be raised up and God would fulfill His covenant with David that a king would sit on his throne (see
2 Sam. 7:25-29). This future King, of course, will be Jesus Christ, the Son of David (
2 Sam. 7:13,
16;
Isa. 9:6-7;
Luke 1:32) who will reign over Israel during the kingdom. In fact, the only Jew alive today who can prove His genealogy and defend His kingship is Jesus Christ! God revealed these truths gradually to His people, but His plan had been settled from the beginning. Neither the Cross nor the church were afterthoughts with God (
Acts 2:23;
4:27-28;
Eph. 1:4). The Judaizers thought that Israel had to "rise" in her glorious kingdom before the Gentiles could be saved, but God revealed that it was through Israel's "fall" that the Gentiles would find salvation (
Rom. 11:11-16). At the time of the Jerusalem Conference, David's house and throne indeed were fallen; but they would be restored one day and the kingdom established. (Ibid)
Messianic Jewish writer Steven Ger gives us his analysis of the
Amos 9:11-12 passage - The Amos passage speaks of God's restoration of the fallen ruins of the tabernacle of David during the
future messianic kingdom. (ED: THAT IS WHAT ONE WOULD CONCLUDE IF THE PROPHECY IS INTERPRETED LITERALLY) The tabernacle was a picture of the house of David, the Davidic dynasty, which metaphorically fell into ruin when the final Davidic king was deposed from power by Babylon (
2 Ki 25:7). Amos promised that, in fulfillment of the Davidic Covenant (
2 Sa 7:12-16), the dynasty of David would be restored to its former glory in the
messianic kingdom.
The Amos passage goes on to relate that when the kingdom of God is established (
IN THE MILLENNIUM), Gentile salvation will be normative. James was not claiming that because of the contemporary salvation of Gentiles that this prophecy was presently being fulfilled, wholly or partially. Rather, James was making a rather sophisticated present day application of a future prophecy. If Gentile salvation is to be understood as normative within the messianic kingdom, it should not be very surprising to anyone if Gentile salvation were to occur somewhat earlier. James was arguing that there was nothing unbiblical or even unanticipated concerning the salvation of Gentiles and that future Gentile salvation in no way precluded contemporary Gentile salvation. The reason James chose this particular passage from Amos is that it shows Gentiles seeking God specifically as Gentiles, and not as Jewish proselytes, masterfully settling the issue of whether circumcision was necessary. James finishes his application with a reminder to recognize that the testimony of Scripture is that the inclusion of Gentiles in God's program has always been part of His plan. Indeed, the Messiah was expected to be a light to multitudes of Gentiles (
Isa. 42:6;
49:6).
There are some who misinterpret James here, believing that he was arguing that this prophecy was fulfilled by first century contemporary circumstances. They erroneously suppose that the ruined tabernacle of David is the church. It is difficult to determine how they could hold this
broadly spiritualized view, as David's tabernacle is described as fallen and in ruins, in need of restoration. Anyone who has read the first fifteen chapters of Acts definitely knows this description not to be true of the victorious first century church! Amos was prophesying of Israel, not of the church, which was an undisclosed mystery within the Hebrew Scripture (
Eph. 3:4-5;
Col. 1:26-27). (Twenty-First Century Biblical Commentary – The Book of Acts: Witnesses to the World)