The old teachers believe that the prophecies of rebuilding of the Temple and the division of the land in Ezekiel 40 thru 48 cannot be fulfilled unless someone identifies every Jew's proper lineage.
They interpreted Malachi's prophecy that Elijah "will sit as a refiner and purifier to indicate that Elijah will return from heaven to supernaturally determine the identities and linage of all the Jews worldwide.
They also believe he will identify who is qualified to be priests or proper Levites in the resumed Temple worship