I was speaking with a Dispensationalist pastor and he made the claim that "the Old Testament always defines Israel as genetic Israel". I mentioned Josh. 8:33 as a counterexample, but he said that that verse is talking about the "assembly of Israel" which should be distinguished from Israel proper, which he claims must be genetically defined. Notes from a friend of mine who attends his church similarly say that "Every time ‘Israel’ is read in the OT, it only ever means a genetic descendant of Jacob (and sometimes it means a subset of this group—northern 10 tribes) – ethnic Jews."
Is there any basis for these assertions? Unfortunately, I was unable to ask the pastor about where he got this idea from. It strikes me as incompatible with Josh. 8:33-35. Exodus 12:42-49 and Ruth 1:16 are also relevant.
I know that Dispensationalists don't all emphasize genetic Israel the way this pastor did, since some of my dispy friends also were surprised by this claim. Are there other Dispensationalist authorities that would make the claim that "Israel" in the Bible is always be defined genetically? If so, what reasoning is used to justify it? If not, then what Dispensationalist doctrine might this pastor have been misinterpreting?