During the millennium, where will the Old-testament saints dwell?

There are three groups of saints in the Bible and all have a different destiny. There are Old Testament saints, the Church, and Tribulation saints. Although many folks from Old Testament times were saved by their faith in a coming Redeemer, they aren’t part of the Church, nor are Tribulation saints. Of these only the Church is called the Bride of Christ and only the Bride has been promised residence in the New Jerusalem. (Rev. 21:9-10) Of all believers in the age of man, The Church is uniquely blessed in this regard.
Old Testament saints will dwell in Israel

Who Lives In New Jerusalem? – Grace thru faith

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