God told Moses that He (God) could have mercy on whomever He wants to show mercy to, and it does not depend on any human desire or effort. When people suffer or see suffering, they blame God. They do not see Him as a merciful God.

Romans 9: 15-16 …For He says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion. 16 It does not, therefore, depend on human desire or effort, but on God’s mercy. 

The scriptures give us an example in the book of Exodus of how God freed His people by repeatedly hardening and having mercy on Pharaoh’s heart. God manipulated Pharaoh’s whole life for His purpose, plain and simple.

Romans 9:17 For Scripture says to Pharaoh: “I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.” 18 Therefore God has mercy on whom he wants to have mercy, and He hardens whom He wants to harden. 

The scriptures ask a popular question that many people ask concerning God’s control of man. “Why does God cause bad things to happen to good people?”

Romans 9: 19 One of you will say to me: “Then why does God still blame us? For who is able to resist his will?”

The scriptures answer the question of God’s control of man by asking another question; who do you, a mere human, think you are, asking God this question?

Romans 9: 20 But who are you, a human being, to talk back to God? “Shall what is formed say to the one who formed it, ‘Why did you make me like this?’”

The question is further answered by the scriptures asking, does God not have the right to do what He wants for unique or everyday purposes?

Romans 9:21 Does not the potter have the right to make out of the same lump of clay some pottery for special purposes and some for common use?