Ἰησοῦς Χριστὸς ἐχθὲς καὶ σήμερον ὁ αὐτὸς καὶ εἰς τοὺς αἰῶνας.It is one of those verses that we know in isolation, yet it was not given as a single unit.
"Jesus Christ: yesterday and today the same—and forever."
In expressing this thought, the skilled author glances backwards at verse 7— Μνημονεύετε τῶν ἡγουμένων ὑμῶν, οἵτινες ἐλάλησαν ὑμῖν τὸν λόγον τοῦ θεοῦ, ὧν ἀναθεωροῦντες τὴν ἔκβασιν τῆς ἀναστροφῆς μιμεῖσθε τὴν πίστιν. These (presumably deceased) leaders who spoke the word of God, preached Jesus to them. Jesus has not changed; Jesus will never change. The Jesus they preached yesterday is the Jesus who lives today, and the Jesus who will reign unto the ages to come.
His thought also carries forwards to verse 9—Διδαχαῖς ποικίλαις καὶ ξέναις μὴ παραφέρεσθε· καλὸν γὰρ χάριτι βεβαιοῦσθαι τὴν καρδίαν, οὐ βρώμασιν ἐν οἷς οὐκ ὠφελήθησαν οἱ περιπατοῦντες. Why should they not allow themselves to be carried away with various and foreign teachings? Because the truth will not change, because Jesus Christ is the same yesterday, and today—and forever.
There's a good deal of truth in Spurgeon's remark: "Rest assured that there is nothing new in theology except that which is false; and that the facts of theology are today what they were eighteen hundred years ago."