I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.- Romans 9:15
Men who are tamers of wild beasts, will frequently, when they have subdued a lion, take a delight in showing to the people how obedient that lion will be to them, and how every word that the lion-tamer chooses to say, it will regard and pay attention to. Thus, when the Lord takes a great sinner, after He has tamed him, removed his heart of stone, and given him a heart of flesh, He desires to show how, without the use of the whip, without a threatening look or an angry word, He causes His enemy to become His diligent servant, His earnest friend. O brethren, it shows the power of love on a man when he is so broken down that the things he sneered at he now preaches with all his might. Surely it showed the power of divine grace when Paul avowed Christ openly and vehemently preached—exposing himself to persecution and death—that same gospel which his soul had previously nauseated; yea, which his zeal, full of bitterness, had kindled to exterminate. God takes great sinners and then appoints and qualifies them to be priests and Levites, in order that He might show the exceeding greatness of His power to usward who believe.
Divine grace, while it comes freely to us, is dispensed freely by God, according to the good pleasure of His will. I should like to hear that text thundered throughout Christendom: “I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.” No man hath any right to the mercy of God. We have all sinned ourselves into outlaw: all the rights we have are the right to be condemned, and the right to be cast into hell; all the rights of man that he can appeal to God for in equity are merged in the wrongs for which he is responsible. If the Lord have mercy, it is His own will to do it: He can withhold it if it pleases Him; so He selects the most degraded, those that have gone farthest from Him, and takes them into His church…He lifteth up the poor from the dunghill, and setteth him among princes, even among the princes of His people. His mercy, power, and sovereignty are displayed when He takes of them to be priests and Levites. ~ C.H. Spurgeon
https://www.blueletterbible.org/comm/spurgeon_charles/sermons/0992.cfm