The Immutability of God is an attribute where “God is unchanging in his character, will, and covenant promises."
The Westminster Shorter Catechism says, ’God is a spirit, whose being, wisdom power, holiness, justice, goodness, and truth are infinite, eternal, and unchangeable.” Those things do not change. A number of Scriptures attest to this idea (e.g. Num. 23:19; 1 Sam. 15:29; Ps. 102:26; Mal. 3:6; 2 Tim. 2:13; Heb. 6:17–18; Jam. 1:17)
God's immutability defines all his other attributes: he is immutably wise, he cannot but be merciful, good, and gracious. The same may be said about his knowledge: God is almighty (having all power), God is omnipotent (having all power), God is omnipresent (present everywhere), God is omniscient (knows everything), eternally and immutably so. Infiniteness and immutability in God are mutually supportive and imply each other. An infinite and changing God is inconceivable; indeed it is a contradiction in definition.