when then shall we say, not injustice on God's part, by no means For he says to Moses 'I will have mercy on whom I have mercy and show grace on whom I am gracious' (Romans 9:14)
Paul quotes the scriptures in Exodus 33:19. If you continue to read the story through Numbers God was not very merciful towards Israel. They complained and they died. They tried offering incense when God did not want them to and they died. They were afraid to go into the promised land so God told them they would all die wondering in the wilderness except for Caleb and Joshua. After God said this many gathered and tried to go into the land to take it as if they had repented for their unbelief but they died. I was really feeling backward reading this but then God reminded me 'I will have mercy on whom I have mercy.'
So then God has mercy on whom he wills and hardens whom he wills (Romans 9:18)
It is a spiritual blessing to be chosen and called by God, to receive blessings from the riches of his grace and to the praise of his glory (Ephesians 1:3-7). Oh to be chosen to receive the riches of God's mercy and loving-kindness and grace (Ephesians 2:3-5). God hardened the heart of Pharaoh, he was a vessel of wrath and destruction so that God's power might be displayed through him (Romans 9:17). Does not the potter have the right to make vessels for honor and some for dishonor? Vessels for wrath and destruction so that others may be vessels of his mercy? "And so that making known the riches of his glory to the extent the vessels of mercy which he prepared beforehand for glory, even us whom he has called..." (Romans 9:23-24).
On the contrary, you have come to Mount Zion, and the city of a living God, the heavenly Jerusalem, and to a myriad of angels, joyfully assembly and the assembly of the first brought forth whose names are inscribed in heaven and to God the judge of all and the spirits of the righteous perfected and to Jesus mediator of a new covenant and to sprinkled blood speaking more effectively than Abel's (Hebrews 12:22-24)
The old covenant Paul calls a ministry of death and condemnation. God before them was like a blazing fire and darkness and gloom and a whirlwind, a blast of trumpets and sound of words which made the people beg to not hear the words anymore. God thundered from the mountain that Moses was on, and the people were terrified and even if an animal approached it would die. Moses himself said "I am terrified and trembling" (Hebrews 12:18-21).
'I will have mercy on whom I have mercy and show grace on whom I am gracious'
Oh, glorious it is to those to whom God has shown mercy and been gracious. In the old God's voice shooked the earth, and yet once more declares the removal of that which can be shaken. "Therefore having received a kingdom unshakable, have grace through which serving God in an acceptable manner with reverence and awe, for indeed our God is a consuming fire" (Hebrews 12:28-29).