For fellow Christians?

I believe Paul is speaking of representatives from all 12 tribes. Remember, in Scripture, "all" does not necessarily mean everyone inclusively. Verse 25 shows that there are two groups, Gentiles and Israel. Israel is not the Church. God made specific promises to Israel, as a nation, that were not for the Church. So, Paul is saying that there will be people from all the tribes who will make it through the tribulation and inhabit the earth in the millennium, who will see and experience the promises fulfilled.