But thou art the same - ואתה הוא veattah Hu, but thou art He, that is, The Eternal; and, consequently, he who only has immortality.

Thy years shall have no end - לא יתמו lo yittammu, "they shall not be completed." Every thing has its revolution - its conception, growth, perfection, decay, dissolution, and death, or corruption. It may be said that regeneration restores all these substances; and so it does in a measure, but not without deterioration. The breed of animals, as well as vegetables, wears out; but God's eternal round has no completion. I repeat it, - what is necessarily eternal is unchangeable and imperishable; all created beings are perishable and mutable, because not eternal. God alone is eternal; therefore God alone is imperishable and immutable.

thou art

Malachi 3:6 For I am the LORD, I change not; therefore you sons of Jacob are not consumed.

John 8:58 Jesus said to them, Truly, truly, I say to you, Before Abraham was, I am.

Hebrews 13:8 Jesus Christ the same yesterday, and to day, and for ever.

James 1:17 Every good gift and every perfect gift is from above, and comes down from the Father of lights, with whom is no ficklenss...

Revelation 1:8,17,18 I am Alpha and Omega, the beginning and the ending, said the Lord, which is, and which was, and which is to come, the Almighty...

years

Psalm 90:4 For a thousand years in your sight are but as yesterday when it is past, and as a watch in the night.

Job 36:26 Behold, God is great, and we know him not, neither can the number of his years be searched out.

The Never Changing One.
"JESUS Christ the same yesterday, and to-day and forever" (Heb. xiii:8). Blessed truth and precious assurance for us poor, weak creatures, yea, among all His creatures the most changing; He changeth not. "For I am the Lord, I change not" (Mal. iii:6). "Of old hast Thou laid the foundation of the earth, and the heavens are the work of Thy hands. They shall all perish, but Thou shalt endure: yea all of them shall wax old like a garment, as a vesture shalt Thou change them, and they shall be changed; …
Arno Gaebelein—The Lord of Glory

Characters and Names of Messiah
For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. S uch was the triumphant exultation of the Old Testament Church! Their noblest hopes were founded upon the promise of MESSIAH; their most sublime songs were derived from the prospect of His Advent. By faith, which is the substance of things hoped for, they considered the gracious declarations …
John Newton—Messiah Vol. 1

Covenanting Predicted in Prophecy.
The fact of Covenanting, under the Old Testament dispensations, being approved of God, gives a proof that it was proper then, which is accompanied by the voice of prophecy, affording evidence that even in periods then future it should no less be proper. The argument for the service that is afforded by prophecy is peculiar, and, though corresponding with evidence from other sources, is independent. Because that God willed to make known truth through his servants the prophets, we should receive it …
John Cunningham—The Ordinance of Covenanting