letteth = holds fast. Greek. katecho, as 2Th_2:6. Supply the Ellipsis by "there is one who holds fast", instead of by repeating the verb "will let". But katecho is a transitive verb, and an object must be supplied too. See all the occurances: 2Th_2:6. If the subject be Satan, the object must be his position in the heavenlies (Eph_6:12), from which he will be ejected by Michael (Rev_12:7-9).
out of the way = out of (Greek. ek) the midst. Compare the same expression in Act_17:33; Act_23:10. 1Co_5:2. 2Co_6:17. Col_2:14.