Romans 9:14-23 is frequently misapplied to how God chooses to save some and condemn others to Hell. The example of Pharaoh is given here, and how God hardened his heart so that eventually he and his people would be destroyed and condemned to Hell. And this is where the analogy of the potter and the clay comes in. What many generally miss is that Pharaoh had already hardened his heart long before God hardened it. Note Exod 5:1-7:
1And afterward Moses and Aaron went in, and told Pharaoh, Thus saith the LORD God of Israel, Let my people go, that they may hold a feast unto me in the wilderness.
2And Pharaoh said, Who is the LORD, that I should obey his voice to let Israel go? I know not the LORD, neither will I let Israel go.
3And they said, The God of the Hebrews hath met with us: let us go, we pray thee, three days' journey into the desert, and sacrifice unto the LORD our God; lest he fall upon us with pestilence, or with the sword.
4 And the king of Egypt said unto them, Wherefore do ye, Moses and Aaron, let the people from their works? get you unto your burdens.
5 And Pharaoh said, Behold, the people of the land now are many, and ye make them rest from their burdens.
6 And Pharaoh commanded the same day the taskmasters of the people, and their officers, saying,
7 Ye shall no more give the people straw to make brick, as heretofore: let them go and gather straw for themselves.
Could Pharaoh have obeyed God at this point? Absolutely
Should Pharaoh have obeyed God at this point? Absolutely
Did he not adamantly refuse to obey, and instead heaped more cruelty on the Israelites because his heart was already hard? Yes
So when we come to Exod 7:13, God hardens his heart. This is true for all sinners, who are all given ample opportunity to repent, but when they resist the Holy Spirit God hardens their hearts. Thus it was with those who were delivered from Pharaoh but then hardened their hearts in the wilderness. Therefore we have this warning in Heb 3:7-19:
7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,
8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:
9 When your fathers tempted me, proved me, and saw my works forty years.
10 Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
11 So I sware in my wrath, They shall not enter into my rest.)
12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
16 For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
17But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
18And to whom sware he that they should not enter into his rest, but to them that believed not?
19 So we see that they could not enter in because of unbelief.
So why would God plead with sinners NOT TO harden their hearts if He had already planned and purposed to harden their hearts and send them to Hell? This passage is another strong evidence that no one is predestined for Hell, and that the Divine Potter does not pre-form some vessels for destruction. He foresees their hardness, but He does not create their hardness, just as He foresees the faith of others and therefore calls them "the elect".