I fully agree that Paul CANNOT contradict himself because it isn't really "Paul's" words but the God-breathed precise words meant to be communicated to us all - for all time. That is why I can say without doubt that the Gospel of God's grace through Jesus Christ has always been one of pure mercy from pure love shown to us by our Creator. It is not dependent on our own goodness - as you rightfully said - we have no innate goodness, at least not what is necessary for eternity in heaven with God. So this is why I also know that our salvation does not depend on our goodness after we have come to saving faith in Christ.
Believing what the Bible says about the basis of our salvation, which is by grace without our own works, I know that anyone who IS saved has been born again and is adopted into the family of God. This adoption is a PERMANENT one and, just as it did not depend on our own merit or work to receive, it does not depend on our works to keep us in the family of God. Nothing Paul, or anyone else in Scripture, said contradicts this essential truth. A person who has come to saving faith in Jesus Christ is PERMANENTLY indwelled by the Holy Spirit and it is GOD'S faithfulness that will complete our journey home.
Can that person fall into sin? Yes, as long as we are still in this natural, old nature we will not be totally free from its influences. Paul understood that as well as anyone. He said:
We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For what I do is not the good I want to do; no, the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it. So I find this law at work: When I want to do good, evil is right there with me. For in my inner being I delight in God’s law; but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. What a wretched man I am! Who will rescue me from this body of death? Thanks be to God—through Jesus Christ our Lord! So then, I myself in my mind am a slave to God’s law, but in the sinful nature a slave to the law of sin. (Romans 7:14-25)
Living a victorious Christian life is not so much one where we NEVER commit a sin, but is one that the closer we get to the Lord; the more mature we grow in our faith; the growth we experience as we learn to resist temptation and choose to do what we know is good and right; the more we learn to allow the Spirit to control our life; the more we are conformed to the image of Christ and our lives glorify our Lord and Savior. If after 25 years as a Christian, Paul could write that he STILL struggled with the old nature, it should not be surprising that we all must walk the same path. We strive towards holiness because God has instilled HIS Spirit within us - it is NOT something we can conjure up of our own strength. So, yes, we will ALL fall at times and we will also get back up again through the grace of God. This falling and getting back up and falling again is all part of being human but with a spirit nature that actually CAN live for God. God knows our frame, he remembers that we are dust, he pities his children and has compassion for us and this is why I know that he will NEVER cast us out or lose us ONCE we are His.
We didn't get saved by being good. We don't STAY saved by being good. It is all the way through about God's grace. This is NOT saying that God condones sin, he doesn't, he disciplines us as a loving Father would his children. But he will never disown us - he can't - he has placed His Holy Spirit within us never to leave us and we are sealed UNTIL the day of redemption. This is the good news, the Gospel, that God wants us to understand.
So, a person who "falls" - if by that you mean commits sin - then he can be forgiven and restored to fellowship with Christ. If you mean a person who "loses" his faith - as the OP says about Charles Templeton - then it boils down to whether or not he ever was truly a believer. It sounds like, from the second article I just posted, he may never have made that assent of faith, but only God knows. The Scriptures assure us that nothing can separate us from the love of God and by that I can know that I have everlasting life through Christ. It is not by works of righteousness which I do, but by His mercy that I am saved and his grace overcomes all.
A person who has come to saving faith in Jesus Christ
Where is "saving faith" in Holy Scripture?
Sorry, but as anyone who reads the Bible knows remaining faithful to Christ as you follow Him carrying your cross isn't "works" salvation or earning salvation.Read the Bible rather than reading what you want to see or what supports the rote, feel good, OSAS doctrine. Everyone who was saved from slavery in Egypt didn't make it to the promised land because they were expected to be loyal and faithful to God in order to receive the reward of entering the Promised land. In particular you might consider the heretic Core and his crowd who insisted that everyone who had come out of Egypt was equally called to be a priest. That refusal to accept the order God ordained for His people angered God the Father who slaughtered Core and his crowd in spectacular fashion.
Core and his pals were making precisely the same claim all Protestant derived churches make and they were cut off. That's exactly why Luther wanted to throw Jude out of the New Testament canon, he knew he knew he was making the same assertion as Core. By your methods of interpretation Core was saved and had nothing to worry about so why were those who sided with him slain? How did they go beyond what the majority of the OSAS crowd asserts is an example of stellar faith and obedience? And what about those who made it all the way to the Promised Land but were not allowed to enter? What's with their being expected to be obedient and faithful in order to cross the river if their behavior didn't matter?
It's clear all through the Old and New Testaments that there are no Harry Potter magic words that free you from every obligation just by saying those magic words. That's what OSAS comes down to, though, "magic words" salvation supported with the Lego Block Method of Scripture Interpretation". The Scripture that says faith without works is dead should be a clue for folks, but I guess when you're off on faith alone as part of your favorite heresy recognizing all Scripture as true is pretty inconvenient.