Difficult Doctrine

Romans 9
21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?

Here, in the 21st Century, I find this passage to be difficult. How should it be explained? How should it be applied?

More From Romans 9
22 What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
23 And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory...

Maybe there are passages that are best not discussed. And, here is the Old Testamen echo of Romans 9:21:

Isaiah 64
8 But now, O LORD, thou [art] our father; we [are] the clay, and thou our potter; and we all [are] the work of thy hand.