The word (correctly) translated "yesterday" is χθές (echthes) which occurs just three times in the NT. BDAG provides two basic meanings for this word as follows:
- the day preceding today, yesterday, eg, John 4:52, Acts 16:35
- time that is past as opposed to now, yesterday, eg, Heb 13:8
Obviously, the second meaning is the relevant one here for Heb 13:8. How far does "yesterday" extend? MacLaren says this:
How far back does this ‘yesterday’ go? The limit must be found by
observing that it is ‘Jesus Christ’ who is spoken of - that is to say,
the Incarnate Saviour. That observation disposes of the reference of
these words to the past eternity in which the eternal Word of God was
what He is to-day. The sameness that is referred to here is neither
the sameness of the divine Son from all eternity, nor the sameness of
the medium of revelation in both the old and the new dispensations,
but the sameness of the human Christ to all generations of His
followers. And the epoch referred to in the ‘yesterday’ is defined
more closely if we observe the previous context, which speaks of the
dying teachers who have had the rule and have passed away. The
‘yesterday’ is the period of these departed teachers; the ‘to-day’ is
the period of the writer and his readers.
The Jamieson-Fausset-Brown Bible Commentary is more precise:
The Jesus Christ (the full name being given, to mark with affectionate
solemnity both His person and His office) who supported your spiritual
rulers through life even unto their end "yesterday" (in times past),
being at once "the Author and the Finisher of their faith" (Heb 12:2),
remains still the same Jesus Christ "to-day," ready to help you also,
if like them you walk by "faith" in Him. Compare "this same Jesus," Ac
1:11. He who yesterday (proverbial for the past time) suffered and
died, is to-day in glory (Re 1:18). "As night comes between yesterday
and to-day, and yet night itself is swallowed up by yesterday and
to-day, so the "suffering" did not so interrupt the glory of Jesus
Christ which was of yesterday, and that which is to-day, as not to
continue to be the same. He is the same yesterday, before He came into
the world, and to-day, in heaven. Yesterday in the time of our
predecessors, and to-day in our age" [Bengel]. So the doctrine is the
same, not variable: this verse thus forms the transition between Heb
13:7 and Heb 13:9. He is always "the same" (Heb 1:12). The same in the
Old and in the New Testament.