God is immutable

[tonychanyt]

Malachi 3:

6a For I the Lord do not change.

The Immutability of God is a divine attribute that God is unchanging in his character. However, the way that he deals with people does change, Joel 2:

28 And afterward, I will pour out my Spirit on all people.

God does not change, i.e., God is immutable. This is an attribute of God Himself. God is perfect. His character does not go through a developmental process.

On the other hand, God's dealing with people changes. That's why there is a difference between the OT and the NT. History changes and develops.

These two are different concepts.

 

[NewLifeInChristJesus]

On the other hand, God's dealing with people changes. That's why there is a difference between the OT and the NT.

I don't think that quite explains why there is a difference between the OT and NT.

This Old Testament passage pretty much explains why there is a difference between the Old Testament/Old Covenant and New Testament/New Covenant:

“Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah— not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the LORD. But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, ‘Know the LORD,’ for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more.” (Je 31:31–34)​

And, more precisely, the first coming of the Lord Jesus, the mediator of the New Covenant, is the one thing that fully explains the difference between the OT and NT:

But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, to redeem those who were under the law, that we might receive the adoption as sons. (Ga 4:4–5)​