Here I'll make it easy for you. What happens before the words: "After this I will return ...". Just read the words in bold below:
"Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name. And to this agree the words of the prophets, as it is written; After this I will return, and will build again the tabernacle of David, which is fallen down; and I will build again the ruins thereof, and I will set it up: That the residue of men might seek after the Lord, and all the Gentiles, upon whom my name is called, saith the Lord, who doeth all these things." [Acts 15]
Now what happens after "After this I will return ...". Just read the above words underlined.
Please note that the "Gentiles upon whom my name is called" is the result of the "visit to the Gentiles [starting at the House of Cornelius] in order to take out of them a people for his name". The visitation comes first and note that the Gentiles after His return are already called by His name --the end result of the visitation.
***Now what happens after “After this I will return ...”. Just read the above words underlined.***
You are stuck in a Dispensational mindset. James has already asserted that the “I will return” had ALREADY happened. Quit reading everything as if it still future to you or even James.
Gentiles hear and believe ~ Peter
The prophets agree ~ James
The LORD returned to his people 2000 years ago. Does the name Imanuel mean anything to you???
The tabernacle was rebuilt.
The gentiles are coming is the proof of the prophetic fulfillment to the council. The same observance by Peter is the same coming of the gentiles in the prophecy. THIS IS WHY JAMES CITES IT.
The "after this" corresponds to Amos' "in that day". The meaning is that "after this" (after the words of the prophecy were given, aka "in that day"), the tabernacle will be rebuilt (Christ is resurrected) so "that the rest of mankind may seek the Lord" (the gentiles then being called).
"In that day" referring to the day when Christ took on human flesh and walked among us, to bring the nations to Himself. There is not one hint of futurism in the passage.
"14 Simon has declared how God at the first visited the Gentiles to take out of them a people for His name. 15 And with this the words of the prophets agree …"
You’re fail to see that James understood everything that was happening in that day was the exact fulfillment of the Amos prophecy, otherwise how could he say it "agreed" if some of it was not even happening?
Reading Amos plainly, if the "tabernacle" was not rebuilt then the gentiles were not being called. But that was clearly not the case.