someone pointed out to me that all through the old testament (septuigent) there is a clear distinction between an "a Isreal" and "the Isreal" meaning "the Isreal" are the loyal followers of Jehovah so I would assume that when Christ came he invited gentiles to join Spiritual Isreal (the Isreal) and it appears from many bible verses that there would be similiar distinction between "a Christian" (some one who calls themselves that but proves otherwise) and a "genuine Christian" I think the Sarah and Hagar situation also point to the 2 groups
Galations 4: "24These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar.
25Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.
26But the Jerusalem that is above is free, and she is our mother"
Its interesting in the picture of the WT article being discussed has a representative of Jerusalem and one of Rome