I don’t have time to go into this deeply and reference scripture, but I can mention a few things that might help. I grew up the son of a Cumberland Presbyterian minister and continue to this day as an elder in that church. My Dad used to say it this way: Once “truly” saved, someone is always saved.
In a prayer right before his crucifixion, Jesus’s prayer says something to the effect that of those who God gave him, not one had been lost. To me, that implies that Judas Iscariot actually never had been given as a true believer in Christ. The question begs touching on apostasy. One can choose to turn away. I think it is clear that Solomon was a true believer in the beginning (read the account of the dedication of the Temple), but he died in apostasy having married forbidden wives and building high places to Molech and Chemosh.
Regarding predestination, I believe the scripture reads that “those he foreknew, he predestined”. God has perfect knowledge. I think he picks his servant based on that perfect knowledge. He knew Jeremiah before he was knit together in his mother’s womb. God knew beforehand what kind of committed servant Saul would become in the person of Paul. He was changed from being a murderer of Christians to one of the most effective servants of Christ ever.
Finally, you can’t ignore scripture that is seemingly contradictory. You have to study, call on the guidance of the Holy Spirit to discern how it all fits together. On the surface, predestination seems to rule out free will. We CPs, even though from a Calvinist tradition, are referred to as “whosoever willers”. John 3:16: “For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.” “Whoever believes” . . . sounds like free will to me. Scripture is clear that Christ died for ALL. We CPers also believe that God makes provision for one dying in infancy; hard line belief in predestination did not accept that since they died before they could make the choice to believe. That was one of the key points when the CP church broke off from the Presbyterian Church US in 1810. But I understand they may have softened their stance on predestination now.
That is a short answer to a deep question. I am sure this will get some flaming, but I believe this to be a pretty accurate lay depiction of the theology.
Your reply interests me. We have been members of a newer Reformed Baptist church that teaches expositorily. We have struggled with reformed teaching on basis of what foreknowledge means. The way they interpret what you say, which is my husband’s understanding, is that foreknowledge doesn’t mean God knows the person would choose to believe because if so that puts the value in the one who believes. That isn’t possible since no one can choose God. I hope I explained what they say.