Psalm 89:34
God never makes a promise or a declaration that He fails to stand by for all eternity (Isaiah 40:8; Titus 1:2). If He says He will do something, He does it (I Samuel 15:29; I Thessalonians 5:24). He confirms by an oath to His followers that His will is immutable (Hebrews 6:17-18). He cannot lie. The mind and the intentions of God never change, and all the hope that we have of our salvation and eternal life is founded on the fact that He and His purpose are fixed (Isaiah 14:24; 46:10-11). He cannot and will not ever break a promise (II Timothy 2:13).
Martin G. Collins
God's Non-Transmittable Attributes (Part Four): Immutability