Well, that is a start. It does some, but little to address issues and associated Scriptures central to your concern on this thread: (1) Are "most Christians [] going to hell"? (2) Is it true "that Christ didn't take care of our salvation [fully] at the cross" but that "it's more up to us that it is up to Him," and (3) "Does a person become an adopted son of God when they're saved, but then become disowned by God when he falls away?" And your response (despite my previous request) does not wrestle with a number of Scripture passages cited or alluded to above or others like them in Scripture. Nor does it go further into Bunyan.
Granted you also doubt affirmative answers to the above paragraph's questions are representative of Scripture (as probably do I, depending on what is meant), and you cite the Prodigal Son parable as supportive of the claim that those adopted in a Christ-saving way do not finally fall away. An elected President does not cease his office when or because he "slips up" (and I think the analogy represents the Scriptures on salvation of us sinners).
Whether Paul in 2 Cor. 13:5 meant what you think he "sounds like" is another question, but it raises one issue of several that think worth articulating:
1) Was Paul in 2 Cor. 13:5 theologically and logically consistent with himself in Rom. 8:31-39? Or more broadly, is the canon of Scriptures coherent? Is a harmonization of the whole possible, even if not always understood?
2) Do true Christians utterly "fall away"/apostatize (esp. after they are truly born again) or does the falling away demonstrate the apostate was never truly a Christian, never truly born again in the first place?
A subsidiary question to # 2 above is whether, once a person is fallen away, can his or her repentance be renewed (can he or she return to being a Christian).
And what does one do with passages of Scripture that suggest a person can fall away/apostatize (alluding for example to Hebrews 6)?
3) How many Christians will prove to be truly Christian "at the end of the day" and how do we know who is a Christian and who is not? (I realize that around the fringes, question # 3 is notoriously difficult, but it seems part of what troubles you.) Are there people who think they are Christians, but are not born again?
And what does one think of hard cases of people we know whose salvation may be in doubt in our minds or who have dropped out of church or renounced Christ? Or is it possible a true Christian can doubt his or her salvation, yet be truly saved?
4) If there is a conflict between what I think must follow and what the Scripture says is true, which do I believe? I ask the question for the principle of the thing, but have in mind particularly divine sovereignty/human responsibility issues.
At this point I could give more of my opinion (or further represent my camp), but I think preferable is to phrase the questions as above in hopes they help you do more of your own homework, so to speak, though I think by now we can both agree that the Scriptures indicate that if persons are to be saved it is because Jesus paid for all their sins with no exception AND that true Christians cannot utterly fall away/apostatize, but persevere in faith and holiness even if they also sin.