God's perfect qualities don't change
His mind (Num 23:19). He is unchangeable in His nature, moral character, perfections, purposes, promises and gifts. He will fulfill His Word including the judgement of sin and sinners. The principle of God's immutable quality (as revealed in Scripture) is that He punishes the wicked and stubborn but blesses and forgives those who are repentant. So although God says those who sin must die, He graciously forgives us when we repent and walk in His ways, granting us eternal life instead of being doomed to eternal death, having passed from death to life (Act 3:19; Rom 6:23; 1 Jn 1:9).
When the Bible speaks of God changing His mind, this refers to His willingness to change His promised course of action when there has been a change in the actions of man such as when the inhabitants of Nineveh repented with the planned judgement postponed (Jnh 3:9,10, 4:2). It is because of the genuine altered human behaviour that there is a different outcome – God has not changed His character or mind but remained faithful to it (Jer 18:7,8, 26:3, 36:3).
If God does not change His planned course of action when there is a corresponding change in human behaviour, He would not be true to His Word, and character of justice and mercy (Gen 18:25; Ps 51:17; Jer 18:10; Ezek 18:21-25). When Jesus became flesh and walked on earth His coming and work were no makeshift action to remedy unforeseen defects in the OT scheme (Jn 1:14). They were but the realization of what God had planned from eternity. "The plans of the Lord stand firm forever, the purposes of His heart through all generations" (Ps 33:11, 110:4; Isa 14:24). He always acts in perfect harmony with the revelation of Himself and His will in Scripture.
God is dependable and His Kingdom cannot be shaken (Heb 12:28). We can confidently trust He will not, in fact cannot, change.
His immutability gives us assurance, comfort and security, providing us with a spiritual and moral anchor in a rapidly changing culture.