God is unchangeable with regard to His will and purpose.
Whatsoever God has decreed, is immutable; whatsoever God has promised, shall be accomplished. He is the Lord Jehovah; therefore He is true to His Word.
As Malachi 3:6 states, “For I am the LORD, I change not; therefore ye sons of Jacob are not consumed.”
We also read in Isaiah 43:13, “Yea, before the day was I am he; and there is none that can deliver out of my hand: I will work, and who shall let it?”
Where we find predictions in Scripture declared and yet not executed, we must consider them not as absolute, but conditional, (such as), so Nineveh shall be destroyed unless the inhabitants repent, which God granted them.