A.T. Robertson Commentary Hebrews 13:8

"Jesus Christ [is] the same yesterday and to-day, [yea] and for ever." — Hebrews 13:8 (ASV)

Jesus Christ is the same yesterday and today, yea and forever (Ιησους Χριστος εχθες κα σημερον ο αυτος κα εις τους αιωνας). There is no copula in the Greek. Vincent insists that εστιν be supplied between Ιησους and Χριστος, "Jesus is Christ," but it more naturally comes after Χριστος as the Revised Version has it. The old adverb εχθες is rare in the N.T. (John 4:52; Acts 7:28; Hebrews 13:8). Here it refers to the days of Christ's flesh (2:3; 5:7) and to the recent work of the leaders (13:7). "Today" (σημερον, 3:15) is the crisis which confronts them. "Forever" (εις τους αιωνας) is eternity as well as the Greek can say it. Jesus Christ is eternally "the same" (1:12) and the revelation of God in him (1:1f.) is final and never to be superseded or supplemented (Moffatt). Hence the peril of apostasy from the only hope of man.