Well, again now, Naomi, my problem with this interpretation is that it changes the subjects being addressed in the passage from the Christians living at Rome 2,000 years ago into all Christians who have ever lived throughout history. I don't read the passage as a bunch of theoretical statements about Christians in general, but as spoken with regard specifically to the Christians in Rome at that time. When he said, "but you became obedient to that form of teaching that was delivered to you," he was talking specifically about them, not us. So any "once ... now" time signatures that may appear in the text don't relate to us, they relate only to them.
Certainly we can glean things from the passage or I wouldn't have started the thread, LoL. But assuming the passage is a bunch of theoretical statements about all Christians is IMO misreading the passage : )
Alright....let me ask you this: when Paul is talking about something like..this:
For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep. For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not precede those who have fallen asleep. For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God. And the dead in Christ will rise first. Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord. Therefore encourage one another with these words. -1 Thessalonians 4:14–18
Do you think that this can only really refer to the Christians then? Do you think we also cannot expect that OUR loved ones asleep in the Lord will be raised also, the we too shall be gathered to him? Of course not, right? This is clear 'theological' information for the whole Church. And it's clearly that because it's about a future that hasn't happened yet.
The problem about pointing at most other NT text and saying it's only particularly relevant for the people back then, is that you strip it all of its theological content, and that is in error. There are times in Paul's writing, as the other books, where they address specific people and events, but in general, Paul is writing to A CHURCH...in other words, to a people of God. And he's writing about theological matters, so that they may be informed about spiritual things. And, as we know God doesn't change, we are therefore free to deduce that what is true for them, is also true for us.
Now this last statement here is where we get into a cruel reality that I think I accept but you reject as being possible with God. The difference between our relationship with God and an earthly father is that He requires us to worship Him as God, and He will have no other gods before Him. Hence a sin like taking the mark of the beast is something He will not overlook either. He is a jealous God, and unlike in the human analogy there are a few things that are paramount to disowning Him. As the scripture says, "If we deny Him, He will deny us." Deny, denounce, and disown are all plausible translations of the word used here in the Greek, and what that means is, He will not simply continue to call His own those who have disowned him before men.
Isn't this funny, because I would have said that yes, there IS a difference between and earthly and heavenly Father....it's the heavenly father that will never let us down.
It's our heavenly Father who sent his Son to rescue us so that we also might be called Sons of God. Yes he is our God, but we also have access to him as our Father, one who Christ is our advocate. And as we know from the verses I keep pointing to, our resting in Christ is secure...on his work, on his love and mercy. It's this love and mercy that will keep drawing us back, not matter how many times we stumble. Our adoption
is sure. It's sure because he 'won't lose us but raise us up' (John 6:39-40), will 'bring us to completion' (Phil 1:6), will 'perfect our faith' (Heb 12:2) and 'glorify us after justifying and sanctifying us' (Rom 8:30), the promise of all this sure because those of us who received the Spirit have it as a 'seal of guarantee of the inheritance promised to us'.
When you thread all of those verses together, and then add the ones that speak against works being able to earn us squat...I cannot, simply cannot, see a biblical basis for your argument, I am sorry.
I think the full passage as I quoted it to Charity explains this better. Let me get it and quote it here.
22 And you shall be hated of all men for my name's sake: but he that endures to the end shall be saved.
23 But when they persecute you in this city, flee into another: for verily I say unto you, You shall not have gone over the cities of Israel till the Son of man be come.
24 The disciple is not above his master, nor the servant above his lord.
25 It is enough for the disciple that he be as his master, and the servant as his lord. If they have called the master of the house Beelzebub, how much more shall they call them of his household?
26 Fear them not therefore: for there is nothing covered, that shall not be revealed; and hid, that shall not be known.
27 What I tell you in darkness, that speak ye in light: and what ye hear in the ear, that preach ye upon the housetops.
28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell...
32 Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven.
33 But whosoever shall deny me before men, him will I also deny before my Father which is in heaven.
34 Think not that I am come to send peace on earth: I came not to send peace, but a sword.
35 For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law.
36 And a man's foes shall be they of his own household.
37 He that loves father or mother more than me is not worthy of me: and he that loves son or daughter more than me is not worthy of me.
38 And he that taketh not his cross, and follow after me, is not worthy of me.
See, I would say that these verses point to characteristics of faithful versus the wicked...a theme we see often throughout scripture. As the faithful we must endure. But...it's not like we don't need encouragement in that. On the flip side...its also not a guarantee that because we're exhorted TO endure, that we will not. In point of fact, I'd say the more encouragement from scripture and the Spirit those of new birth get, the more likely we are to NOT fade in our faith. In fact, I'd argue (and clearly have

) that there are many, many passages that speak towards a Christian's inability to truly fall away as you suggest, simply because when we look at the base of it all, it's not us holding our salvation together; it never was, it never will be. That is the work of someone more mighty then us.
Likewise, for the rest of this passage, we see a direct contrast between those who 'confess God before men, and those who just don't, and the punishment they receive. But, there is nothing in this passage that demands an understanding that the 'person' being discussed has to be singular and that they had to have started as a person of faith who has wavered and failed. Indeed, Christ is, as he often did in his parables, holding up the 'two types of people' in the world and stressing the worldly 'difficulty' of taking up the cross to follow Christ in this life, but the very real danger to ones eternal soul in the next life to deny him in this one.