The primary function of the doctrine of predestination is to explain why some individuals respond to the gospel and other do not.
I can see how John could have developed his logic. If all humans are equal, to start with, it is reasonable that all would make the same good or bad choices, all equal, if they were all equally made by God. Yet, we see people making very unequal choices, showing their character is not all equally good or bad.
And we see how very evil people have made exact opposite choices about Jesus. So, the only explanation can be that God made the difference.
This is what I can see that John could arrive at, from such a starting question. Predestination, then, for him, would not be a starting doctrine, but a conclusion.
If I may, I would care to share how I developed. I was not really concerned about if predestination is true. But after I made a pass at getting saved by Jesus, I fell and failed and was ashamed and embarrassed and frustrated, not knowing what happened.
Then I went through a process of discovering how I could really not control my own self. And I could see I was at God's mercy. Then I was brought to Romans 9:21 which says >
"Does not the potter have power over the clay, from the same lump to make one vessel for honor and another for dishonor?" (Romans 9:21)
I was so undone, that I was fully ready to believe this meant God not only chose what He would do with me, but also He is my Potter who controls what will become of me for all eternity. So, it seemed to me, then, that Romans 9:21 goes beyond what John Calvin has been saying, about predestination > there is not only God choosing, but God in control.
This, though, does not mean He was actually causing me to be the wrong way, but He was in control of which way things in me were allowed to go in me or to leave while He would bring in what is good.
So, I stopped and was still, and discovered what became of me. I sensed that God was doing what He wanted, and I would be doing the discovering.
And Philippians 2:13 says,
"for it is God who works in you both to will and to do for His good pleasure." (Philippians 2:13)
To me, this meant I do not work my own will to do what God wants, but I need how God alone in my will is able to produce the willing . . . of what to believe, what to do, and all He pleases. So, I considered myself totally at the mercy of God.
More and more I have seen the love meaning of Biblical predestination >
If we all were made "from the same lump", we all have come from what was used to make our Apostle Paul and Saddam Hussein and predatory priests and Adolph and me and you . . . all "from the same lump" . . . all starting from the same basic human stuff . . . stuff which could have turned into a predator or a pastor!! So, I need to be humble about if I have not become like Adolph or Saddam or others. And have compassion on any and all other people, however they are turning out.
And our Apostle Paul says what is the main purpose of predestination >
"For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren." (Romans 8:29)
So, our destiny in Jesus is not mainly about us and going to Heaven and being prospered by God and even loved by our Father; but first of all, I now see, is how our Father so is pleased by Jesus that He desires to have many children who are like Jesus so pleasing to Him.
So, this needs to be what gets our attention the most . . . not only who is making choices and who is controlling. Our egos can be hung up about who is in control and who makes choices. But we need how our Father in us is changing us to be like Jesus; and this is a major part of Christianity, and Paul says this is what he has been laboring for, in Colossians 1:28-29.