God is someone you can trust. He isn’t fickle. He doesn’t change with the weather. His love is constant and His character is trustworthy and He is true to His word. One man said that God’s unchangeableness means that God is unchanging in his being, perfections, purposes, and promises, yet God does act and feel emotions and he acts and feels differently in response to different situations (Systematic Theology, Grudem, p163).

God himself is unchangeable:

25 In the beginning you laid the foundations of the earth,
and the heavens are the work of your hands.
26 They will perish, but you remain;
they will all wear out like a garment.
Like clothing you will change them
and they will be discarded.
27 But you remain the same,
and your years will never end.

Psalm 102:25-27

This is one of the striking differences between The Creator and his creatures. We, God’s creatures, are always changing – progressing (or regressing) – but never stagnant. God on the other hand is always himself – He doesn’t change.

God’s purposes don’t change as well. The Bible says in Psalm 33:11

The counsel (purpose) of the LORD stands forever, the thoughts of his heart to all generations

Psalm 33:11

This is why the Bible describes God as a Rock! What is more unchangeable than a rock?!! Only God!