“After this I looked, and behold, a great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands, and crying out with a loud voice, ‘Salvation belongs to our God who sits on the throne, and to the Lamb!'”
How is Revelation 7:9-10 fulfilled? By the consummation of all things, when Christ returns and gathers his people into one congregation. By the worship of the church now, because Christ has already raised all his people together with him before the Father as one worshiping congregation.
Revelation 7:9-10 is fulfilled, now, and will be fulfilled later, because Christ has accomplished salvation. Revelation 7:9-10 is not accomplished by local congregations embodying any kind of demographic diversity, any more than it is fulfilled by wearing white robes or holding onto palm branches. Local congregations participate in the fulfillment of Revelation 7:9-10 by being faithful to our savior, through joining our voices together in worship with the heavenly, spiritual, and eschatological congregation. How is Revelation 7:9 modeled by the local church? By faithfulness to our confession of praise to our God and Lamb.