This is an alternative to "Spiritual Israel" and the 144,000 being one and the same.
Two scriptures might support the subset idea:
First, Revelation 7:4-8 states that the 144,000 are "out of" the 12 symbolic tribes of "Spiritual Israel"--12,000 "out of" each tribe. This would indicate to me that "Spiritual Israel" is a larger body than the 144,000 (whether a literal number or not).
Second, the New Covenant is said to be between God and "Spiritual Israel," entitling its members to be children of God and brothers of Christ. Once this covenant was initiated, Jesus then made a separate Covenant for a Kingdom with his apostles that would entitle them to "sit on thrones and judge the twelve tribes of Israel," a function more closely associated with the 144,000 (Luke 22:28-29; Revelation 20:4).
Might the Covenant for a Kingdom be to the 144,000 what the New Covenant is to "Spiritual Israel"? The more expansive New Covenant would apply to the overall group, but the more limited Covenant for a Kingdom would apply to the subset group.
The one hope for all Christians of being children of God and brothers of Christ would still apply, but a specific function would be assigned to those designated to be of the 144,000.
Just a thought. Any ideas?
DNCall