Yes you could indeed spend a few posts on Romans 6:15-22. And you would get it wrong yet one more time. The key to that passage is in a correct understanding of verse 21, namely what does Paul intend with the talk of the potter's, i.e., God's, "right over the clay" for " honorable use" or "dishonorable use". First, what is the "clay" here? It is not the mass of humanity, the human race in general, as the Calvinists want to say. The entire discussion is concerned with the totality of Israel, from which God makes the two derivative groups, physical Israel and spiritual Israel. This should be reasonably clear based upon the first verses in the chapter. Paul makes it clear that he is talking about his "brothers, his kinsmen according to the flesh" (v.3). Israel as a nation, is in the hand of God as a potter molds clay. I would remind you of Jeremiah 18:1-12, where the clay in the potter's hand is the people of Israel.
From the same lump of clay, says Paul, the potter has the right to make pieces of the pottery that are very different in their nature and use. And here as in Jeremiah, God makes it clear that this potter-clay relationship does not mean that he arbitrarily determines the destiny of the nation. He declares that he tailers his final decision regarding any nation of kingdom to the way it responds to his warnings (Jer 18:7-12):
Jer 18:7 If at any time I declare concerning a nation or a kingdom, that I will pluck up and break down and destroy it,
Jer 18:8 and if that nation, concerning which I have spoken, turns from its evil, I will relent of the disaster that I intended to do to it.
Jer 18:9 And if at any time I declare concerning a nation or a kingdom that I will build and plant it,
Jer 18:10 and if it does evil in my sight, not listening to my voice, then I will relent of the good that I had intended to do to it.
Jer 18:11 Now, therefore, say to the men of Judah and the inhabitants of Jerusalem: 'Thus says the LORD, Behold, I am shaping disaster against you and devising a plan against you. Return, every one from his evil way, and amend your ways and your deeds.'
Jer 18:12 "But they say, 'That is in vain! We will follow our own plans, and will every one act according to the stubbornness of his evil heart.'
With respect to God's dealing with individuals, we need to understand that Paul is, in verses 20-21, answering the objector in verse 19. for his presumptuousness in talking back to Go. The metaphor of the potter and his clay is a generic warning applicable to anyone who presumes to do the same in any sort of circumstance. Even so, we know that Paul intends that it be applied to "his brothers, his kinsmen according to the flesh", i.e., Israel. We know that from the way he continues the metaphor in verses 22-24. God has endured with much patience the nation Israel, in order to make known the riches of his glory for the salvation brought to the world through Jesus Christ.
The Potter-Clay is metaphor is God's arbitrary choosing for service. Paul shows us that God used ethnic Israel to produce spiritual Israel. He shows us that it is God's right to choose and use people or nations or kingdoms without saving them.