It has been said that God cannot change his mind or alter his plans once decreed. Two verses in particular point to this:
Numbers 23:19 (NIV)
God is not human, that he should lie, not a human being, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?
1 Samuel 15:29 (NIV)
He who is the Glory of Israel does not lie or change his mind; for he is not a human being, that he should change his mind.
But there are clear instances where God in fact changed his mind and/or altered his declared course of action. Consider these passages:
Genesis 18:23-32 (NIV)
Then Abraham approached him and said: “Will you sweep away the righteous with the wicked? 24What if there are fifty righteous people in the city? Will you really sweep it away and not spare the place for the sake of the fifty righteous people in it? 25Far be it from you to do such a thing—to kill the righteous with the wicked, treating the righteous and the wicked alike. Far be it from you! Will not the Judge of all the earth do right?” 26The Lord said, “If I find fifty righteous people in the city of Sodom, I will spare the whole place for their sake.” 27Then Abraham spoke up again: “Now that I have been so bold as to speak to the Lord, though I am nothing but dust and ashes, 28what if the number of the righteous is five less than fifty? Will you destroy the whole city for lack of five people?” “If I find forty-five there,” he said, “I will not destroy it.” 29Once again he spoke to him, “What if only forty are found there?” He said, “For the sake of forty, I will not do it.” 30Then he said, “May the Lord not be angry, but let me speak. What if only thirty can be found there?” He answered, “I will not do it if I find thirty there.” 31Abraham said, “Now that I have been so bold as to speak to the Lord, what if only twenty can be found there?” He said, “For the sake of twenty, I will not destroy it.” 32Then he said, “May the Lord not be angry, but let me speak just once more. What if only ten can be found there?” He answered, “For the sake of ten, I will not destroy it.”
Exodus 32:10-14 (NIV)
Now leave me alone so that my anger may burn against them and that I may destroy them. Then I will make you into a great nation.” 11But Moses sought the favor of the Lord his God. “Lord,” he said, “why should your anger burn against your people, whom you brought out of Egypt with great power and a mighty hand? 12Why should the Egyptians say, ‘It was with evil intent that he brought them out, to kill them in the mountains and to wipe them off the face of the earth’? Turn from your fierce anger; relent and do not bring disaster on your people. 13Remember your servants Abraham, Isaac and Israel, to whom you swore by your own self: ‘I will make your descendants as numerous as the stars in the sky and I will give your descendants all this land I promised them, and it will be their inheritance forever.’” 14Then the Lord relented and did not bring on his people the disaster he had threatened.
Numbers 14:12-20 (NIV)
I will strike them down with a plague and destroy them, but I will make you into a nation greater and stronger than they.” 13Moses said to the Lord, “Then the Egyptians will hear about it! By your power you brought these people up from among them. 14And they will tell the inhabitants of this land about it. They have already heard that you, Lord, are with these people and that you, Lord, have been seen face to face, that your cloud stays over them, and that you go before them in a pillar of cloud by day and a pillar of fire by night. 15If you put all these people to death, leaving none alive, the nations who have heard this report about you will say, 16‘The Lord was not able to bring these people into the land he promised them on oath, so he slaughtered them in the wilderness.’ 17“Now may the Lord’s strength be displayed, just as you have declared: 18‘The Lord is slow to anger, abounding in love and forgiving sin and rebellion. Yet he does not leave the guilty unpunished; he punishes the children for the sin of the parents to the third and fourth generation.’ 19In accordance with your great love, forgive the sin of these people, just as you have pardoned them from the time they left Egypt until now.” 20The Lord replied, “I have forgiven them, as you asked.
Numbers 16:21-24 (NIV)
“Separate yourselves from this assembly so I can put an end to them at once.” 22But Moses and Aaron fell face down and cried out, “O God, the God who gives breath to all living things, will you be angry with the entire assembly when only one man sins?” 23Then the Lord said to Moses, 24“Say to the assembly, ‘Move away from the tents of Korah, Dathan and Abiram.’”
2 Kings 20:1-6
In those days Hezekiah became ill and was at the point of death. The prophet Isaiah son of Amoz went to him and said, “This is what the Lord says: Put your house in order, because you are going to die; you will not recover.” 2Hezekiah turned his face to the wall and prayed to the Lord, 3“Remember, Lord, how I have walked before you faithfully and with wholehearted devotion and have done what is good in your eyes.” And Hezekiah wept bitterly. 4Before Isaiah had left the middle court, the word of the Lord came to him: 5“Go back and tell Hezekiah, the ruler of my people, ‘This is what the Lord, the God of your father David, says: I have heard your prayer and seen your tears; I will heal you. On the third day from now you will go up to the temple of the Lord. 6I will add fifteen years to your life. And I will deliver you and this city from the hand of the king of Assyria. I will defend this city for my sake and for the sake of my servant David.’”
These passages should not be (mis)construed that God is fickle, that God will always change his mind, nor that we can compel God to change his mind. God is sovereign, the very definition of which means God enjoys executive prerogative to do whatever he wants without lessening his divinity. But that having been said, there is no harm in praying for God’s mercy. Even Jesus prayed for God to change his mind: “if it is possible, may this cup be taken from me” (Matt 26:39 NIV). But Jesus also qualified his request: “yet not as I will, but as you will.”
As I suggested in a prior post, prayer is as much about changing our perspective(s) as it is anything else. God’s nature is to give us good things, which might or might not be what we ask. As long as we don’t try to back God into a corner, demanding that he do x or change y, then we can expect God to respond gently to our petitions even if those petitions are to change something that he will not grant.