From the KJV
1 Corinthians 3:
5 If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?
17 If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.
1 Corinthians 5:
5 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife.
2 And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you.
3 For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed,
4 In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ,
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.
Once a person has been imbued with the Holy Spirit, they are a temple of God. If the person rejects being led by the Spirit and instead continues to be led by his flesh, that is, his sinful nature, what that person is building upon is sin and not a foundation of Jesus, who is Spirit. That whole structure is bound for the flames of eternal damnation. Very little of the person's spiritual life will exist due to nothing having been built upon the foundation of the temple of God residing within.
There are two aspects to salvation. The first is that we be born again. Without that experience, we remain dead in trespass and sin. Then we need to be delivered from the habits and memories of the old nature that we received from Adam. This is a lifetime work. We need to be set free from sins and hindrances (Hebrews 12:1), the self life (Luke 9:24) and we need our minds renewed (Romans 12:2)
Some Christians are unwilling to let go of sins, such as bitterness, resentments, hatred and wrong relationships. 1 Corinthians 6:9 tells us what will disqualify us from the Kingdom of God. So God will deal with the man's flesh so that his born again spirit will cease to be oppressed by his self will. Such a person's spirit will go to God, but he will not experience the rewards of the kingdom. In the case of the Corinthian sinner, he repented and was restored. God does this out of love. As Lord Jesus said, the kingdom of God should be our number one priority. If we have to give up something for the sake of the Kingdom, God will reward us with something much better. (Matthew 6:33)
It is very important to note that Paul says that the person is saved, they have their life, they just have nothing else. This is not a heaven or hell judgement.
The doctrine of Justification by Faith is not based on your works but rather the finished work of Jesus Christ. It is by grace, and these 2 passages from 1 Corinthians actually both illustrate that.
In 1 Corinthians 3, Paul is talking about earning rewards, and there are more rewards than just eternal life, because as we can see, it is possible to get eternal life, but nothing else. Escaping destruction, as if escaping from a house on fire with nothing but the clothes on your back. That's the image Paul gives.
11 For other foundation can no man lay than that is laid, which is Jesus Christ.
12 Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble;
13 Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is.
14 If any man's work abide which he hath built thereupon, he shall receive a reward.
15 If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
In both cases both of these men get eternal life. In the first case it's a man who's justified by Faith and he has evangelized, he's made disciples, he's done good works, he's been a good ambassador of Christ, edifying, teaching, preaching, spreading the gospel. He's building on the foundation that Jesus saved him from his sins. None of that work gets him into eternal life, that's all the work of Christ that does that, but this man's work FOR Christ, is not ignored, it's rewarded. Paul gives examples of crowns (that is symbols of authority) and Jesus Himself gives the example of ruling over cities as a reward.
In the second case.. this man is still justified by faith, but he hasn't put it to good use, nothing that he has done has been working for the kingdom.
all his works are shown to be worthless for the kingdom, but his sins are still forgiven so he still has his life. He's just not going to have any authority, hey, someone has to be ruled on the new earth right?
In the second case, you have a brother, that is someone who believes on Jesus Christ, but they've backslidden into sin, really bad sin, sin that requires Church discipline throwing him out of the church and not fellowshipping with him anymore.
In fact they're letting Satan have his way, maybe even killing the man, because in death, he won't sin anymore.
But Paul is offering, that if he was really a believer and just not showing it because of his backslide, his spirit would still be saved.
Think about the story of Lot. Lot was as backslidden as you could imagine, pitching his tent towards Sodom, and he had some authority in that city, he knew what kind of people he was associating with, and he still did it anyway, even offering up his daughters to be raped by all the men of the city.
No way is this guy behaving like Christ right, couldn't be saved right? He should have gotten nuked by God like all the other exceedingly sinful people in that city.
But he was saved from the destruction of that city and used as an example for us for good reason, that it's forgiven sinners that are saved, not the self righteous.
So, in conclusion, neither passage in 1st Corinthians is about backsliding Christians going to hell, but rather, about backslidden Christians being chastised (even to physical death) for their sin, and losing out on the rewards that Jesus wanted to give them on top of eternal life, but because they didn't work for the kingdom they forfeit their reward and will have to just settle for eternal life.
I know that sounds crazy because everyone kind of sees eternal life and resurrection as the rewards themselves... but there's more to it than that. Eternal life and resurrection are gifts, they're really the lowest common denominator when it comes to heaven. They are the bare bones minimum.
and that's what 1 Corinthians 3 teaches.