Isaiah 45:25 says, "In the LORD all the offspring of Israel shall be justified and shall glory."
According to the OT, the "offspring of Israel" are not all physical descendants of national Israel, but only those who will be justified in the LORD and glory. So, who are they? In Isaiah 45:22, just a couple of verses earlier, we read, "Turn to me and be saved all the ends of the earth; for I am God and there is no other." Anyone who turns to the LORD for salvation will be saved, justified, and inherit glory. And, that group is the offspring of Israel.
Evidently, only those who believe, even in the OT, were truly the "offspring of Israel" and all Israelites, on a biblical definition, will be justified and shall glory.