Zechariah 6:15
And they that are far off shall come and build in the temple of the LORD, and ye shall know that the LORD of hosts hath sent me unto you. And this shall come to pass, if ye will diligently obey the voice of the LORD your God. And they that are far off shall come and build in the temple of the LORD, and ye shall know that the LORD of hosts hath sent me unto you. And this shall come to pass, if ye will diligently obey the voice of the LORD your God.
The visions to which this promise belongs were given to encourage God's people while they were rebuilding the temple amid uncertainty, assuring the builders of God's compassion and support for restoring His house. Those who are far off shall come and build in the temple of the Lord. Their coming and building will confirm that the Lord of hosts sent His messenger to His people. This promised outcome will come to pass only if the people diligently obey the voice of the Lord their God.
Those who are far off are the Gentiles. The temple is the true Christian church that Christ builds. Christ, the Branch, builds the temple of the Lord and is its foundation and foundation stone.
The apostles fulfill this by gathering people from the Jewish and Gentile world as living stones for that building. Paul's work in the Gentile quarry carries this out by bringing out Gentile converts as valuable stones to lay upon the foundation stone Jesus Christ. By coming into contact with that living stone they also become living stones, built together into a holy temple and a dwelling place for God.