Thank you for your informative post, but I must take issue with your exegesis....
First we must understand the basis of our salvation. Is salvation based on what we do?
Rom 4:5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
Titus 3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;
These scriptures, and countless others, make it clear that salvation is not based on what we do for God. In fact, it is just the opposite. Our salvation is based on what God did for us (Rom 5:8; 1 Cor 15:3,4; 2 Cor 5:21).
This is, at best, selectively quoting the scriptures. First, there are numerous texts that say we need to do things in addition, to what God has done for us to stay in the grace of God: Baptism (Acts 2:38, 22:16, 1 Peter 3:21, Mark 16:16), Works (10:17-31, James 2:12-26, Matthew 25:31-46) and enduring until the end (Matt 24:13). All of these verses either mention salvation, justification, the forgiveness of sins etc. and none of them mention faith alone. While they are all done through God's grace, all of these verses necessitate human action.
Since we can't do anything to be saved, how can we be expected to do anything to stay saved? The scriptures teach that we are secure in Christ and that nothing, not even our own unbelief, can separate us from the love of God once we have trusted in Christ's sacrifice for our sin. (Rom 8:33-39; Phil 1:6; 2 Tim 2:11-13)
Because point one is demonstrably false then point two must also be false, and the biblical record bears this out. First lets look at the verses you quoted:
33Who will bring any charge against those whom God has chosen? It is God who justifies. 34Who is he that condemns? Christ Jesus, who died – more than that, who was raised to life – is at the right hand of God and is also interceding for us. 35Who shall separate us from the love of Christ? Shall trouble or hardship or persecution or famine or nakedness or danger or sword? 36As it is written:
"For your sake we face death all day long;
we are considered as sheep to be slaughtered."[1] 37No, in all these things we are more than conquerors through him who loved us. 38For I am convinced that neither death nor life, neither angels nor demons,[2] neither the present nor the future, nor any powers, 39neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.
I must point out that all the things that St. Paul mentions here are things that are
external to the believer. The text does not say "neither adultery, nor murder, nor any other sin can seperate us from God." In fact, in his first epistle to the Corinthians, he mentions things that can specifically seperate us from God:
9Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders 10nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God. 11And that is what some of you were. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.
(1 Cor 6:9-11)
Paul emphatically declares that although the Corinthians were involved with those activities, but they are now christians and these things are not to be done by them anymore. Unfortunately that's exactly what they had been up to, which necessitated Paul's letter to them. If they continued in sin, they would not inherit the kingdom.
being confident of this, that he who began a good work in you will carry it on to completion until the day of Christ Jesus.
This verse cannot possibly be used to support Eternal Security for several reasons. First, Paul nowhere says that the Phillipians had a sure ticket to heaven because he only says that he is
confident that the good work will carry it on to completion. Had he known of the Phillipians eternal security, he would've made the language absolutely certain that they would all be in heaven.
The rest of the letter bears this out because he warns the Phillipians to "work out their salvation with fear and trembling." Had they been eternally secure, this verse would make absolutely no sense whatsoever. Moreover, Paul says explicitly that his salvation had not been attained yet:
I want to know Christ and the power of his resurrection and the fellowship of sharing in his sufferings, becoming like him in his death, 11and so, somehow, to attain to the resurrection from the dead.
12Not that I have already obtained all this, or have already been made perfect, but I press on to take hold of that for which Christ Jesus took hold of me. 13Brothers, I do not consider myself yet to have taken hold of it. But one thing I do: Forgetting what is behind and straining toward what is ahead, 14I press on toward the goal to win the prize for which God has called me heavenward in Christ Jesus. Phillipians 3:10-14
Paul also explains the uncertainty of his salvation to the Corinthians:
Do you not know that in a race all the runners run, but only one gets the prize? Run in such a way as to get the prize.
25Everyone who competes in the games goes into strict training. They do it to get a crown that will not last; but we do it to get a crown that will last forever. 26Therefore I do not run like a man running aimlessly; I do not fight like a man beating the air. 27No, I beat my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize. (1 Cor 9:24-27)
Paul's explanation of the apostasy of Israel in the following chapter also gives this warning greater depth. Although the Israelites had been "baptized into Moses" (like Christian baptism) and ate the same spiritual food and drank the same spiritual drink that we drink (Holy communion), but despite this fact, God was not pleased with most of them and only Caleb and Joshua made it through the desert. In addition, to say that these people weren’t “true believers” is ludicrous. This is because the faith of this generation is mentioned in Hebrews 11 along with a group of the greatest Old Testament saints who were no doubt true believers(Hebrews 11:29). Paul also mentions doubt of the salvation of another individual in his second letter to Timothy:
16May the Lord show mercy to the household of Onesiphorus, because he often refreshed me and was not ashamed of my chains. 17On the contrary, when he was in Rome, he searched hard for me until he found me. 18May the Lord grant that he will find mercy from the Lord on that day! You know very well in how many ways he helped me in Ephesus. (2 Tim 1:16-18)
Although Onesiphorus was no doubt a good man of God (since he was dedicated to Paul despite the fact that virtually everyone had abandoned him), but Paul abstains from using language of Onesiphorus’ final destination. All of this lends a lot more credibility to conditional security than unconditional security.