Hi Barry
Thanks for the question. I will try to explain to you why I believe that a person during this time period cannot lose his/hers salvation. I will address some of the verses you presented.
First one would be 1 Corinthians 15:2
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
 
God's Word transaltion says this
2 In addition, you are saved by this Good News if you hold on to the doctrine I taught you, unless you believed it without thinking it over.
 
If a person believes the Gospel then what happens?
Ephesians 1:13
13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
 
So after a person believes he is SEALED with the Holy Spirit. The Holy Spirit is attached to the person and will not leave him. This is the only dispensation (the dispensation of grace) where we are actually sealed. Must we produce fruits during this time?
Romans 4:5
5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
 
Ok now back to 1 Corinthians 15. There were people in Corinth who "said" they were believers but yet they did not believe in a resurrection of the dead. Paul is simply telling them that you have to believe in the Gospel to be saved. What is the Gospel? Paul goes on to tell them.
1 Corinthians 15:3-4
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;4 And that he was buried, and that he rose again the third day according to the scriptures:
 
Well the resurrection is part of the Gospel of Grace we are to believe to be saved. Evidently some in the church did not believe the entire Gospel so in essence they were never sealed with the Holy Spirit to begin with. They may have believed "in God" but they obviously had a problem believing in the entire Gospel. Paul is saying if you did not believe the entire Gospel, The death, burial, and resurrection fo Jesus then you believed in vain. It is no good to you. For had they believed the entire Gospel they would have been sealed. If you would know them by their fruits then surely Paul would have known who believed and who did not. Remember Paul is talking to "babes in Christ" and not mature Christians. He goes on to tell them in this chapter that the believers will be caught up in the twingling of an eye. He is telling them their sinful,mortal bodies will be changed. But if you do not believe in the resurrection you will not be a part of it. Nothing to do with the believer losing his salvation. Verse 14 tells them exactly what I believe.A person has to believe in the resurrection or that person is never sealed nor saved.
! Corinthians 15:14
14 And if Christ be not risen, then is our preaching vain, and your faith is also vain.
 
Now let me answer 1 Timothy 1:19-20
19 Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck:20 Of whom is Hymenaeus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme.
 
This passage has NOTHING to do with a person losing his salvation. He is talking specifically about Hymenaeus and Alexander. He delivered them to satan. What does this mean? We can look in 1 Corinthians 5 and see where Paul uses this same terminolgy.
1 Corinthians 5:5
5 To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
 
Look at God's Word translation
5 hand such a person over to Satan to destroy his corrupt nature so that his spiritual nature may be saved on the day of the Lord.
 
When Paul is delivering a person to satan he is removing them from the fellowship of the church. They are still saved. The verses above only prove a person who truly believes the Gospel and I mean trust ONLY in the gospel is saved. If Paul could judge their salvation by their fruits he could have just picked out the Corinthians who bore no fruit and again explained to them the Gospel, the death, burial, and resurrection of Jesus.
 
Now the verses you quoted from the Old Testament were true but that was for a "different" time period. They were never sealed witht the Holy Spirit in that time period.
As for Hebrews and 1 and 2 Peter, those books are written to the tribulation saints. They too will not be sealed and if they do not prove their faith when "tested" or "tempted" (I honestly do not understand the difference) then they may be in real danger. If they are tempted or tested during this period it will more or less be to take the mark of the beast. If a person does that during this time period he will NOT be saved. Even if he had believed in the death burial and resurrection of Jesus. During the tribulation it does not matter who you are, if you take the mark of the beast it is over for you.
 
When God completed the Bible he also put in the books for the people living after the rapture of the church. These books are very clear when you realize who they are talking to. Starting with the book of Hebrews and going through Revelation you will find those books very interesting as long as you apply them to who they were written to. Romans through Philemon are written for us today. God put the plan for the Jews and the Kingdom on hold whenever the nation of Israel rejected Jesus as their Messiah. He has blinded them. After the rapture the book of Hebrews is the book that starts giving the instructions for living. We can learn a lot from the entire Bible but we need to be careful about which time period the Bible is referring to or a person can get very confused and end up teaching a "perverted" Gospel which Paul says that person is cursed.
 
I hope this clears up what I believe.
 
thanks
Mike W