“For I am the LORD, I do not change; therefore you are not consumed, O sons of Jacob.”
“Every good gift and every perfect gift is from above, and comes down from the Father of lights, with whom there is no variation or shadow of turning.”
Jesus Christ is the same yesterday, today, and forever.”
Malachi 3:6, James 1:17, Hebrews 13:8
From the passages quoted above, it is an inescapable conclusion that God does not change. Yet, in studying the scriptures, we find that God does in fact change. In the Old Testament we find the passage in Genesis 18:17ff., where the Lord changed his course of action based upon the pleadings of Abraham. Likewise, in Deuteronomy 9:13ff., Moses recounts to the children of Israel how the Lord was set on more than one occasion to destroy them, yet he himself had pled for them, and the Lord changed his mind. In the New Testament we also have examples of the Lord changing his actions, based on the appeal of man, for example in Matthew 15:22ff., with the Canaanite woman whose daughter was demon-possessed; his original course of action was to ignore her, yet because of her faith, he did as she desired.
It is also evident that during the course of the ages God has changed his covenant with man – from the patriarchal, to the Mosaical, to the Christian.
Therefore, as it is “impossible for God to lie”, there must be a sense in which God does change, and a sense in which he does not. If God does change his covenants with man, and if he will change his proposed actions, then the sense in which he does not change must be in regard to his character.
We are under the New Testament, as far as the covenant which is in force at this time (and until the end of time; for this is the last covenant until He returns for the Judgment). We may use the Old Testament for examples, for Paul wrote, “For whatever things were written before were written for our learning, that we through the patience and comfort of the Scriptures might have hope,” Romans 15:4; God hasn’t changed in character, only in the covenant which is in force. We may see an example of this in Zephaniah 3:5, “The Lord is righteous in her midst, He will do no unrighteousness.” We may go to the Old Testament for examples of how he will deal with us, by seeing how he dealt with the men of old. We may not go there in order to bind things of the law of Moses upon man today – we are not under the law of Moses, but of Christ – but we may in fact learn how he dealt with transgression, how he upholds his part of things, etc., as these things deal with the character of God, in which he does not change.
As an example, Jesus has said that “true worshippers” will worship “in spirit and in truth,” John 4:23. This is not new to the New Testament; God also desired the same in the Old Testament, cf. Joshua 24:14. We must worship in spirit - in sincerity, and in truth - according to the covenant in effect at the present time. So, then, when we see Nadab and Abihu, this is in fact also a warning to us – for though they may have been sincere in their worship (we don’t know), yet it was not in accordance with truth. And, as this was unacceptable to God, and caused their deaths, we may be assured that the same will happen to us if we transgress the truth regarding worship that is in effect today. We may not be struck down on the spot, but will not death still await us? For the warning of God still stands, “By those who come near me I must be regarded as holy, and before all the people I must be glorified,” Lev 10:3. In disregarding the command of God, they neither regarded him as holy, nor glorified him before the people; so Moses explained to Aaron. The method (law) of approach has changed, but God has not.
We may further confirm this, if we recall the following, “Anyone who has rejected Moses' law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace? For we know Him who said, "Vengeance is Mine; I will repay," says the Lord. And again, "The Lord will judge His people." It is a fearful thing to fall into the hands of the living God,” Heb 10:28-31; cf. Luke 11:29-32, Matthew 11:20-24.