Immutable is a theological term to describe God's unchanging nature and character. The concept of immutability arises from several biblical passages.

With a contrast to changing aspects of earth and the heavens, the psalmists portray God as unchanging in nature and unaffected by time. The implication is that God is perfect, because He cannot improve or decay.

Of old You founded the earth,
And the heavens are the work of Your hands.
Even they will perish, but You endure;
And all of them will wear out like a garment;
Like clothing You will change them and they will be changed.
But You are the same,
And Your years will not come to an end. (Ps 102:25-27)

By juxtaposing the inconsistencies of human beings, the prophets emphasize that God's word and mind does not change.

God is not a man, that He should lie,
Nor a son of man, that He should repent;
Has He said, and will He not do it?
Or has He spoken, and will He not make it good? (Num 23:19)

Also the Glory of Israel will not lie or change His mind; for He is not a man that He should change His mind. (1 Sam 15:29)

That God does not lie is a declaration of confidence to trust His word and plan – for eternity! The implication that God's word is timeless means that the Bible does not change with time nor out of date. It is just as relevant today as it was in the past.

Remember the former things long past,
For I am God, and there is no other;
I am God, and there is no one like Me,
Declaring the end from the beginning,
And from ancient times things which have not been done,
Saying, ‘My purpose will be established,
And I will accomplish all My good pleasure;
Calling a bird of prey from the east,
The man of My purpose from a far country.
Truly I have spoken; truly I will bring it to pass.
I have planned it, surely I will do it. (Isa 46:9-11)

Paul, a bond-servant of God and an apostle of Jesus Christ, for the faith of those chosen of God and the knowledge of the truth which is according to godliness, in the hope of eternal life, which God, who cannot lie, promised long ages ago, but at the proper time manifested, even His word, in the proclamation with which I was entrusted according to the commandment of God our Savior, (Tit 1:1-3)

Every good thing given and every perfect gift is from above, coming down from the Father of lights, with whom there is no variation or shifting shadow. In the exercise of His will He brought us forth by the word of truth, so that we would be a kind of first fruits among His creatures. (James 1:17-18)

Jesus, the Son of God, is explicit about the enduring nature of His words, and the writer of Hebrews affirms the unchanging nature of His word.

Truly I say to you, this generation will not pass away until all these things take place. Heaven and earth will pass away, but My words will not pass away. (Matt 24:34-35)

Jesus Christ is the same yesterday and today and forever. (Heb 13:8)

For Believers the immutability of God provides the assurance that the Bible is immutable and that God's moral standard is absolute. Moreover, God's love is not capricious, and the promise of salvation and eternal life is real and assured.