Hebrews 6:4-6 (New International Version)
4It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5who have tasted the goodness of the word of God and the powers of the coming age, 6if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Hebrews 6:4-6 (GOD’S WORD Translation)
4Some people once had God’s light. They experienced the heavenly gift and shared in the Holy Spirit. 5They experienced the goodness of God’s word and the powers of the world to come. 6Yet, they have deserted Christ. They are crucifying the Son of God again and publicly disgracing him. Therefore, they cannot be led a second time to God.
Can you lose your salvation? Once you have believed and accepted that Jesus died as a payment in full for our sins, can you fall back into a sinful life and be lost forever? This has always been a hard passage for many, and it has always been a source of anxiety for Christians.
The Bible seems to state so clearly that once we come to the saving knowledge of Jesus Christ that God will not ever let you fall away. Read these words from Jesus:
John 6:37-40 (New Living Translation)
37 However, those the Father has given me will come to me, and I will never reject them. 38 For I have come down from heaven to do the will of God who sent me, not to do my own will. 39 And this is the will of God, that I should not lose even one of all those he has given me, but that I should raise them up at the last day. 40 For it is my Father’s will that all who see his Son and believe in him should have eternal life. I will raise them up at the last day.”
Paul also touched on this in these verses from his letter to the Romans:
Romans 8:35-39 (New International Reader's Version)
35 Who can separate us from Christ's love? Can trouble or hard times or harm or hunger? Can nakedness or danger or war? 36 It is written,
"Because of you, we face death all day long.
We are considered as sheep to be killed." —(Psalm 44:22)
37 No! In all these things we will do even more than win! We owe it all to Christ, who has loved us.
38 I am absolutely sure that not even death or life can separate us from God's love. Not even angels or demons, the present or the future, or any powers can do that. 39 Not even the highest places or the lowest, or anything else in all creation can do that. Nothing at all can ever separate us from God's love because of what Christ Jesus our Lord has done.
There has long been a dispute between those who believe in "eternal security" and those who don't. Those on the "eternal security" side state that once you have been saved, even if you backslide (fall back into a sinful lifestyle), you will always be saved from going to hell. The other side believes that if you fall back into a sinful lifestyle, you can lose your salvation and end up in hell. The verses above in Hebrews certainly seem to confirm this.
This is how I see it: First of all, if your conscience is bothering you whether or not you have passed the point stated in the verse above, then you are probably alright. Not that you should be testing God by continuing to sin, but I believe that if you truly believe with you whole being that Jesus Christ died for your sins, you have accepted His work on the cross, that God has forgiven you, and from that point on you will seek to lead a new life God's way, you are going to spend eternity with Him. The Bible is too full of promises for the repentant believer for it to be any other way.
The important word here is "repentance", turning away from your former life of sin. Once you have accepted Christ's work of salvation, a believer should always be continually working on living the right way with God's help, and working on Jesus being the Lord of their life:
Philippians 2:12-13 (New International Version)
12Therefore, my dear friends, as you have always obeyed—not only in my presence, but now much more in my absence—continue to work out your salvation with fear and trembling, 13for it is God who works in you to will and to act according to his good purpose.
Second of all, it defeats God's purpose for our lives if we are constantly wondering if we are saved or not. Christ died once in for all for the sins of all the people; past, present and future. Once we have accepted this, we need to move on, working daily to know God better and seeking to please Him. Doing otherwise, puts the focus on our works, rather than on what Jesus did on our behalf.
Ephesians 2:8-10 (New International Version)
8For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God— 9not by works, so that no one can boast. 10For we are God's workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do.
So if you are worried whether or not you will lose your salvation, good. Stop doing the things you shouldn't, turn away from it and start leading the life that God expects you to live.