1 Corinthians 9:27 - But I discipline my body and bring it into subjection, lest, when I have preached to others, I myself should become disqualified. Note: Paul knew that he had to disciplne himself or else he would have been disqualified.
He's talking about being disqualified to receive the prize of the imperishable crown (received in eternal life) as the 2 previous verses in the bible explain. Instead of "disqualified," in the KJV it has "castaway." It's usually translated "reprobate." It also means "rejected, not standing the test, not approved, that which does not prove itself such as it ought." Other times the word (the Greek word "adokimos") is used in the NT, it's talking about the lost (2 Cor. 13:5-7 , 2 Tim. 3:8 , Titus 1:16).
So since Paul was aware of the danger of not being approved and becoming lost, we also need to be careful.
1 Corinthians 10:1-12 - ... I do not want you to be unaware that all our fathers were under the cloud, all passed through the sea,... and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ. But with most of them God was not well pleased, for their bodies were scattered in the wilderness. Now these things became our examples, to the intent that we should not lust after evil things as they also lusted. And do not become idolaters as were some of them. As it is written, "The people sat down to eat and drink, and rose up to play." Nor let us commit sexual immorality, as some of them did, and in one day twenty-three thousand fell; nor let us tempt Christ, as some of them also tempted, and were destroyed by serpents; nor complain, as some of them also complained, and were destroyed by the destroyer. Now all these things happened to them as examples, and they were written for our admonition, upon whom the ends of the ages have come.
THEREFORE LET HIM WHO THINKS HE STANDS TAKE HEED LEST HE FALL. Note: The Greek word for "fall" is "pipto." It also means "to fall under judgement, come under condemnation, to fall out, fall from i.e. shall perish or be lost, to fall from a state of uprightness, to perish i.e. come to an end, disappear, cease." That's definitely a loss of salvation.
1 Corinthians 15:1,2 - Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, IF YOU HOLD FAST THAT WORD WHICH I PREACHED TO YOU -- unless you believed in vain. ~ Note: The Greek word for "if" is "ei." It's a primary particle of CONDITIONALITY meaning "if, whether." So clearly it makes being saved CONDITIONAL. The Greek word for "in vain" is "eike," and it has the idea of failure. It also means "without a cause, inconsiderably, without success or effort." So obviously our believing in the word of God can fail. We must keep believing the word of God and hold on to that word by letting it be real in our lives.
Galatians 5:4,7 - You have become estranged from Christ, you who attempt to be justified by law (O.T. law); you have fallen from grace... You ran well. Who hindered you from obeying the truth? ~ Note: The KJV has "Christ is become of no effect unto you." The Greek words are "apo" and "katargeo." "Apo" also means "of any kind of separation of one thing from another by which the union or fellowship of the two is destroyed." "Katargeo" also means "destroy, loose, to cause a person or thing to have no further efficiency, to cause to cease, put an end to, do away with, annul, abolish." The Greek word for "fallen from" is "ekpipto." It also means "to fall from a thing, to LOSE it, to perish," and "to fall powerless, be without effect" regarding "of the divine promise of salvation." Obviously those who "have fallen from grace" were running well (the race of faith, 2 Tim. 4:7) and obeying the truth at some earlier time, but then they fell from grace.
Philippians 2:12,13 - Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure. ~ Note: The Greek verb for "work out" is "katergazomai." It also means "to perform, accomplish, achieve, to fashion i.e. render one fit for a thing." Either way you look at it, this is something that we are told to do. While Christ purchased our salvation and offers it to us as a free gift, still there is a part that we must do if the salvation is to be completed in our case. The way "trembling" ("tromos") is used with "fear" ("phobos") here, it means "used to describe the anxiety of one who distrusts his ability completely to meet all requirements, but religously does his utmost to fulfill his duty." This Scripture is a warning that salvation is never ultimately guaranteed, but must be worked out. If we are guaranteed entry into heaven, then it makes no sense for the need to work out our salvation. The loss of rewards doesn't produce "fear and trembling," for how bad can heaven be? But this Scripture is not about rewards, because it says "salvation." So hold on to your salvation through faith and work it out with fear and tremblng before our awesome God who is the One energizing us to respond to His grace.
Colossians 1:22,23 - ...to present you holy, and blameless, and above reproach in His sight -- IF INDEED YOU CONTINUE IN THE FAITH, grounded and steadfast, AND ARE NOT MOVED AWAY from the hope of the gospel... ~ Note: Again, this is conditional, with the phrase here "IF indeed you continue." The Greek word for "continue" is "epimeno." It also means "to stay at or with, to tarry still, still to abide, remain, to persevere." This Scripture implies that one can stop having faith and move away from the truth of God's word. DON'T STOP! Stay strong in the faith. It is our responsibility to remain abiding in Christ.
1 Thessalonians 3:5 - For this reason, when I could no longer endure it, I sent to know your faith, lest by some means the tempter had tempted you, and our labor might be in vain. ~ Note: The word for "endure" is "forbear" in the KJV. It also means "to cover over with silence, to keep secret, to hide, conceal (of the errors and faults of others)." So Paul couldn't keep quiet any longer. He had to ask them about their faith. The word for "tempter" and "tempted" also means "to try or test one's faith, virtue, character, by enticement to sin." I know God is not playing games with us by putting this in His word if it can never really happen. I know through experience, and because it is right there in His word.
1 Thessalonians 3:8 - For now we live, if you stand fast in the Lord. ~ Note: Surely the word "if" makes this conditional. The word for "stand fast" also means "to stand firm, to persevere, to persist, to keep one's standing." This is definitely our responsibility to be firm and persevere in the Lord.
2 Thessalonians 2:3 - Let no one deceive you by any means; for that Day (the return of Christ) will not come unless the falling away comes first... ~ Note: The falling away is not a so-called Pre-Trib rapture as some people think. The Greek word for "falling away" is "apostasia." It also means "to forsake, defection, apostasy." This is about apostate Christians who rebel, defect, and fall away from God. In the tribulation, when more severe persecution comes, many believers will grow cold and abandon the faith. Only those who continue until their end will be saved.