Does James's quotation of Amos 9:11–12 in Acts 15:16–18 mean he is applying the words to the present church age, or to the millennium?
This text is from a letter written by David Gooding in 1988.
The way I would go about understanding James's argument here is first to ask: 'What is "the tabernacle [or tent] of David"?' It is not the temple, nor the city of Jerusalem, nor the nation of Israel: it is the royal house and line of David.
Secondly, in what sense had David's royal house fallen? In the sense that, ever since the exile, the Davidic dynasty had been swept away, and there were no kings of the house and line of David (see the perplexity of the psalmist in Psalm 89:38–40).
Thirdly, at what point did James think the house of David was, or would be, built again? Would it have to await the millennium? The following Scriptures appear, to me, to settle the question:
Luke 1:68–70. God has already raised up a horn of salvation in the house of his servant David (as he indicated through the Prophets).
Acts 13:34–37. In raising up Jesus from the dead, now no more to return to corruption, God has already given him the sure mercies promised to David.
Acts 2:30–35. David, foreseeing that of the fruit of his loins God would set up one upon his throne, spoke of the resurrection of Christ. Christ has already risen and ascended: he has been made both Lord and Christ (see v. 36).
See also 2 Timothy 2:8; Revelation 22:16 (NB: he speaks to the churches as the root and offspring of David).
Conclusion so far: If great David's greater Son is already risen from the dead and seated far above all rule and authority and power and dominion and every name that is named (Ephesians 1:21), I would find it difficult to believe that the royal line and house of David is still 'fallen down'. Has not God's promise to build David a house (2 Samuel 7:11, 13, 16) been already fulfilled in Christ, according to Hebrews 1:5?
Then again there is the question of the thought flow of James's argument. He cites Amos in order to support what Peter has done and said regarding the salvation of the Gentiles (Acts 15:7–11). Peter's point has been that God has taken out of the Gentiles 'a people for his name' (Acts 15:14)—without those Gentiles having to be circumcised and become part of the physical nation of Israel. To many Jews and to some professing Jewish Christians (see Acts 15:5), the idea that Gentiles could be 'a people for God's name' without being circumcised or becoming members of Israel was alarmingly novel.
'No it's not novel', says James (see Acts 15:18): 'God prophesied through Amos long ago that this would happen: the rebuilding of the house of David would be followed by the conversion to Christ of multitudes of Gentiles 'upon whom God's name is called' (see Acts 15:17). In other words, it seems to me that 'the Gentiles upon whom my name is called' must be the same group as 'a people for his name' (Acts 15:14) if James's citation of Amos is going to prove that what Peter has done and said agrees with the words of the prophets.
Finally, 'first' in Acts 15:14 is not to be taken as meaning that what Peter did happened first and then 'after that' (see Acts 15:16) God would raise up the fallen house of David. The phrase 'after these things' in Acts 15:16 belongs to Amos's prophecy and must be understood in its context in Amos.
