Another important consideration in this issue is that the Bible teaches that salvation is by faith, not by works. Because Gods standard is perfection, and we have all fallen short of that, we can never earn salvation by works. But this also means we cannot keep our salvation by works. Salvation has to be by faith in Christ from start to finish.
If keeping salvation depends upon our performance, how good does our performance have to be? At what point will the holy God say, "I could tolerate those previous sins you had committed but that sin you have just committed is too much, and youre out"? Either our salvation depends on our performance, in which case one sin is enough to disqualify us, or our salvation depends on Christ, in which case no sin can disqualify us because His death pays for our sins. If keeping our salvation depends on our being good enough, then none of us are good enough because God will not say which sins, or a certain number of sins, are tolerable. The grace of God and the blood of Christ will either cover all our sins or none, but it will not do a part way job. Also, if we have to keep our salvation by being good those in heaven would have something to boast about, but the Bible says no one can boast, because salvation is completely by faith (Rom 4.1-8; Eph 2.8-9).
Continuing in sin
Now this all needs clarification because people get the wrong impression, namely that someone can be saved and live a life of sin. If someone is prepared to live a selfish sinful life in light of the grace of God and the sacrifice of Christ, it evidences that they have never experienced salvation in reality. The Bible teaches that the believer cannot continue to live in sin, the reason being that when someone believes on the Lord Jesus then the Holy Spirit comes in to dwell within them to empower them and motivate them to live a new life. The believer is born again and receives a new nature that loves holiness and hates sin. This new nature is not instead of the old sinful nature that we all were born with, but it is as well as the old nature, so every Christian experiences a struggle between the two natures, and whichever nature the Christian feeds will be the one which manifests itself. The presence of the Holy Spirit ensures that a believer cannot and will not continue in a course of sin indefinitely, and they will not be happy when they embark on a sinful lifestyle. God will deal with them, even making them miserable to bring them to repentance.
But what about the verses which appear to say that believers can lose their salvation? Remember that we have looked at passages of Scripture which deal directly with the subject of the believers security, and those passages teach explicitly that a child of God can never be lost. Thus all other passages need to be interpreted in the light of these clear verses. Often people look at Old Testament narrative passages and use statements they read there to teach that a believer can be lost. This is not a sound way to study the Bible. The passages in Scripture intended to teach us on a particular subject should be the passages we use to form our beliefs on that subject, and then we interpret narrative sections in a way which is consistent with those beliefs.
When we come across verses that seem to suggest that salvation can be forfeited there are a couple of things to bear in mind. Often such verses are not stating the condition upon which people are saved, but the indispensible indications that people are saved. For example, if someone is living in a continued, settled course of rebellion against God, and they are happy in doing so, it indicates that they have never been born again, because the Spirit of God would not let them continue to live such a life. This is a theme John emphasises in his First Epistle where he points out that if we have been born into Gods family (i.e. born again) then it is obvious that we will exhibit characteristics of our Father, and if such "family traits" are missing then the conclusion is that the person has never been born again.
The book of Hebrews is a book from which many people take verses to show that a believer can lose their salvation, but Hebrews is in a very specific context that needs to be understood. The writer is addressing Jews who have been presented with the gospel and have seen the miraculous sign gifts that God gave to the early Christians to validate their message. (Abandoning all the God-given rituals of Judaism was something that they could not do without miraculous proof. That is one reason why the apostles had miraculous powers, and why we do not see the same things today.) These Jews had become convinced of the truth of the gospel, yet there were some who were tempted to turn away from fully embracing Christ as Lord and Saviour and go back to the Jewish rituals. The writer is warning them that if they sin wilfully in this way they are turning to a system that offers no salvation and no true sacrifice for sin. If they refuse the truth they have, it will be impossible to bring them to repentance. The "wilful sin" in Hebrews refers to those Jews who, having seen the truth of the gospel, did not take that final step, and turned away from it to obsolete Judaism. It is nothing to do with a Christian deliberately committing a sin. Just think if it was saying that sinning wilfully disqualifies a person from salvation, no one would be saved, because every Christian has sinned wilfully.
Shallow, insincere "faith"
It is also true that people can have a false faith that is shallow or insincere. Praying the sinners prayer or coming to the front during an appeal etc. does not give someone a ticket to heaven. Many people make a response to the gospel but do not repent (i.e. change their attitude towards sin). They can respond to the gospel without having an appreciation of their guilt and helplessness, or they can "accept Christ" as a friend, helper, or example, but not, as the Bible demands, as Lord and Saviour. Such people may show an initial change, but soon revert to their old lifestyle. The Bible teaches that it is not the case that these people have lost their salvation but rather that they never actually had it! Concluded.