Election.
Lets look at some verses backward and ask a few questions.
Known unto God are all his works from the beginning of the world. Acts15:18
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 15:17
Who is the residue of men? When does this take place?
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: Acts 15:16
After what? Who will return where and from whence? Why at that time would the tabernacle of David need rebuilding? Just what is this about?
Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name. Acts 15:14
Why did God not just take all of them? Why leave a residue? Was it actually God taking out of them, electing, or was he offering all of them and letting them chose whether or not they wanted to jump on board? Do we get to elect to believe, have faith? Is the choice ours or is God taking out a people for his name?