Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee. In perfect peace - שלום שלום shalom, shalom, "peace, peace, "i.e., peace upon peace - all kinds of prosperity - happiness in this world and in the world to come.
Because he trusteth in thee "Because they have trusted in thee" - So the Chaldee, בטחו betacho. The Syriac and Vulgate read בטוח batachnu, "we have trusted. "Schroeder, Gram. Hebrews p. 360, explains the present reading בטוח batuach, impersonally, confisum est.