A.T. Robertson Commentary Hebrews 9:28

"so Christ also, having been once offered to bear the sins of many, shall appear a second time, apart from sin, to them that wait for him, unto salvation." — Hebrews 9:28 (ASV)

Once (απαξ). "Once for all" (verse 26) as already stated.

Shall appear a second time (εκ δευτερου οφθησετα). Future passive indicative of οραω. Blessed assurance of the Second Coming of Christ, but this time "apart from sin" (χωρις αμαρτιας, no notion of a second chance then).

Unto salvation (εις σωτηριαν). Final and complete salvation for "them that wait for him" (τοις αυτον απεκδεχομενοις). Dative plural of the articular participle present middle of απεκδεχομα, the very verb used by Paul in Php 3:20 of waiting for the coming of Christ as Saviour.