"First and Last" as title of Jehovah occurs three times in Isaiah.
“Who has performed and accomplished it, Summoning the generations from
the beginning? ‘I, the LORD, am the first, and with the last. I am
Gill summarizes the intended meaning here about Jehovah's eternal unchanging nature as the idiom of "generations" depicts. He is before the first generation and will long outlast the last generation.
I the Lord, the first, and with the last; I am he; the immutable
Jehovah, the everlasting I AM, the Alpha and the Omega, the beginning
and the ending, the first and the last; all which is said of Christ,
and is the person here speaking, Revelation 1:8, phrases expressive of
his eternity and deity; he is the first and the last in God's
thoughts, purposes, and decrees; in the covenant of grace; in the
creation of all things; in the salvation, justification,
sanctification, adoption, and glorification of his people; and in the
church, above and below
Barnes and the Pulpit commentary arrive at the same conclusion. Further, according to V5, 6 the title also encloses God's omnipotence and omniscience and His claim to be the "I AM" (LXX).
“This is what the LORD says, He who is the King of Israel and his
Redeemer, the LORD of armies: ‘I am the first and I am the last, And
there is no God besides Me.
Again from Gill (and others similarly) we see here an expression of God's working throughout eternity but His existence is well beyond and unchanging - He is always God.
I am the first, and I am the last; the first cause and last end, of
all things in nature, and providence, and grace; all things are of
him, through him, and from him; all things were made by him in
creation, and for his pleasure they are and were created; and all
things are disposed of in his providence for his own glory; and he is
the first in reconciliation, justification, and salvation, and all are
to the glory of his grace: or this is a periphrasis of his eternity,
who is from everlasting to everlasting, without beginning or end, the
Alpha and Omega; the same is said of Christ, Revelation 1:8, and all
the other characters before mentioned agree with him:
According to V7 & * the title is also connected to God being "the Rock" and thus, inique.
“Listen to Me, Jacob, Israel whom I called; I am He, I am the first, I
am also the last.
Ellicott says this:
(12) Hearken unto me, O Jacob.—The prophet is drawing near to the end
of the first great section of his book, and his conclusion takes the
form of a condensed epitome of the great argument of Isaiah 40-47,
asserting the oneness, the eternity, the omnipotence, the omniscience
Gill also reaches a similar conclusion:
I am he, I am the first, and I also am the last; the everlasting I AM,
the immutable Jehovah, the Alpha and Omega, the beginning and the
ending, the first cause and last end of all things; phrases expressive
of the self-existence, supremacy, eternity, and immutability of
Christ, Revelation 1:8, and what is it that such a sovereign, eternal
and unchangeable Being cannot do?
This idea is also connected to God's supreme creativity according to V13
Thus, God's self-proclaimed title, "First and Last" expresses His (a) eternity, (b) the fact that God is before all things and will exist long after the last of things, (c) the only (unique) God, the Great "I AM" (d) His immutability, as well as omnipotence, omniscience, omnipresence, etc.