Several of the world's denominations teach that Christians cannot sin so as to become lost; that is, there is no way that a true convert of Christ he cannot go to hell may sin, and thereby, receive eternal damnation - he cannot go to hell after having once been saved. This is popularly spoken of as the doctrine of "once saved, always saved".
On one extreme, this doctrine is expressed as, "True Christians will not sin as they lack the desire and the capability to sin, because their allegiance to Christ has changed their nature". At its other extreme, this doctrine is expressed as, "No matter how grievous, the sins of Christians will not be held against them in the final judgement". Those who hold this last view believe that Christians can do wrong, but they also believe that Christ will forgive them of any, and every, sin so that they will be blameless on the day of judgement.
You researched this question to find, not what man teaches, but what the Bible teaches about the possibility of Christians falling from the way of truth, and becoming lost.
The most clear treatment of this subject in the New Testament is found in the second chapter of II Peter. Study this entire chapter carefully. I quote now verses 20 and 21, which best summarize its teachings, "For if after they have escaped the pollution of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it to turn from the holy commandment delivered unto them".
Unquestionably the people spoken of here were once Christians, for the writer speaks of them as those who had, "(quote) escaped from the pollution of the world through the knowledge of the Lord Jesus Christ" (unquote). Unquestionably these same people were, again, entangled in worldly pollution, that is, they had fallen from the way of the truth, and had (again) become sinners. Also, unquestionably they are subject to condemnation for these sins, for the verse says that their final state is worse than their initial state, that is, they are worse off than before they were saved. Thus, the doctrine of "once saved, always saved", is not supported by the scriptures, for the Bible here shows that Christians can sin, and that they may receive punishment for those deeds.
All verses of the Bible agree with all other verses when the subject is the same, and when the rule, or law of God, is the same. Also, when the rule, or law, is consistent, clear verses serve to explain less clear verses about the same subject. Said in a different way, the Bible never contradicts itself, and the Bible is its own best interpreter. Remember, God cannot lie, and God is not the author of confusion. Study Hebrews 6:18, and I Corinthians 14:33. To illustrate this, we can study verses about baptism (for instance) in the New Testament books written by the apostles John, Paul, and James, and expect agreement. But, we cannot necessarily expect agreement between the Old Testament writings of Moses and the New Testament writings of Paul concerning (say) worship; Moses' laws were in effect until Jesus' death ended them. See Col. 2:14. Paul's writings, however, are a part of the will of Christ, His New Testament. The New Testament is our guide today.
In I John 3:9, we read, "Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God". If this verse teaches, "once saved, always saved", it is contrary to II Peter 2:20 and 21 (which we read previously). Thus, a contradiction is forced into the scriptures which cannot exist. However, apparent conflict disappears when we see that the writer is stating the obvious fact that Christians cannot sin and remain faithful followers of the Lord. This is verified by the very same writer in the very same letter where the apostle, John, in I John 1:8-10 says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. If we say that we have not sinned, we make him a liar, and his word is not in us".
Here, again, we find an inspired writer telling true believers, Christians (including himself) that they can sin, and that they can do unrighteousness. More deeply, John is here instructing Christians how they can find forgiveness for their sins; Christians are to confess their sins and they will be forgiven them. This instruction is not for sinners who have never become Christians. Christians are citizens of God's kingdom, and must abide by its rules. People who are not citizens in the kingdom must first become so before its laws apply to them. We suggest that you find how to become a citizen of God's kingdom through study the message, "What Must I Do To Be Saved?" and "Are We Saved By Faith Only?".
Belief in, "once saved, always saved" may seem justified by John 10:27-29, "My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand". If this scripture proves, "once saved, always saved", the scriptures contradict each other, and this, of course, is an impossibility. To harmonize the scriptures (to make them agree, one with another), we must notice that Jesus did not say that his sheep could not leave of their own accord; He said that no man could pluck them away.
Each Christian may leave Christ, if he chooses; but unless the individual wishes to go, no one else can make him leave. Notice in verse 27, Jesus said that His sheep, "know me and follow me", and thus, following, and continuing to follow the Savior is a condition that we must fulfill if the promise, "neither shall any man pluck them out of my hand" is to be kept.
And so, in addition to those things necessary to become saved (namely, believing the word of God, believing that Jesus is the Christ, the Son of God, repenting of (or changing away from) a sinful life, and being baptized into Christ for the remission of sins, we have an additional requirement, that of being faithful unto death (Revelation 2:10).
"Once saved, always saved" is a dangerous doctrine: The Christian who does fall away (upon seeing his sinful life) may conclude that he was never saved and become doubtful of the power of God; another may live a sinful life, and become lost, while believing that God is unmindful of the sins being committed. Either way can result in eternal death for the sinner's soul, and it would be such a useless death, for (as we have seen), the doctrine of "once saved, always saved" is wrong. The Christian who sins must confess those sins to God, who will forgive them.