The Bible teaches that there is one church and gates of Hades will not overcome it. My understanding is that it is talking about the eternal church. In Luke 17:21, it consists of the saved ones only. This church will continue to exist till the end of time. But the local congregation is made up of wheat and weeds, just as the nation of Israel was a representation of the people of God. The nation of Israel failed hopelessly, as do many local congregations. Ephesians 3:20 is true, however; it is pointing to the eternal church and not the local congregations. Please compare the parable of the vineyard of the book of Isaiah to the one in Matthew 21... -- Doctor Vukeya 

Right, Matthew 16 does not promise that the local church will last forever. Now Luke 17 is not referring to the church, but to the kingdom. The two are not the same. (Please see the various articles at this website.) The church is Jesus' plan, not a human idea or afterthought. It is to be glorious, triumphant, and it is the bride of Christ. At the end of time the church, which is the bride of Christ, will meet its groom.

I think the Lord is saying that even when people die (and are in the waiting place, Hades) they will still be saved and see God. That is, death cannot rob people of their salvation--a fear apparently entertained by some in the Thessalonian church.

Yes, I agree with you that Jesus is speaking about the church in general, or the saints as a whole. He does indeed also suggest that many will remain in the fellowship even when their hearts are far from God. His taking over Isaiah's Parable of the Vineyard (in the NT parallel passages) supports this understanding.