This is a very legitimate question. I mean really, your driving down the street and someone cuts you off so you cut them off and... you sin. OK, you realize it, feel bad about it and repent. Did you lose your salvation? Did you lose it for an hour before you repented? Did you just luck out that you didn't die during that hour? What if you get mad at God and tell him to go take a hike. Do you then lose the salvation you once had?
I think the answers are, again, in the Bible. Hebrews 12:2 tells us Jesus is the author and finisher of our faith and salvation. So we really have nothing to do with it but only have to believe and accept it. That is our choice. To believe in Jesus or to not believe.
Jesus tells us some interesting things in John 3:14-18
"As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in him should not perish, but have eternal life.
For God so loved the world, that he gave his one and only Son, that whoever believes in him should not perish, but have eternal life.
For God didn't send his Son into the world to judge the world, but that the world should be saved through him.
He who believes in him is not judged. He who doesn't believe has been judged already, because he has not believed in the name of the one and only Son of God."
Use Strong's Concordance to check out the meaning of the words "Have" and "Eternal" in the original Greek. If you have or posses something that is eternal, by definition you cannot lose it. If you could, you either did not posses it in the first place OR it was not eternal. This is what Jesus said. If you believe in him, Jesus, you have eternal life. This is something only you and God can know, and you only about yourself. If anyone ever tries to tell you that you can lose your salvation and you know that you have believed in what Jesus said, simply tell them to go argue with Jesus and not with you.
Don't be fooled. Many so called "religious leaders" want you to jump through this hoop and that hoop. If you don't, you'll lose your salvation. Jesus doesn't make you jump through any hoops. Jesus has done all the heavy lifting for you. Jesus said:
"Come unto me, all ye that labour and are heavy laden, and I will give you rest. Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls. For my yoke is easy, and my burden is light."
Now some people say that if you can't remember the exact moment when you were saved, your not saved. This does not make a lot of sense. This is like saying if you don't remember the first word you ever spoke, you can't speak. If you can't remember the day you were born, you weren't born! As far back as I can remember the old song comes to mind "Jesus loves me this I know, for the Bible tells me so." Am I a sinner? Yes! Do I need a Savior? Yes! Am I saved? Yes! Do I remember the exact moment I was saved? No. If you are trusting in Jesus plus nothing for your Salvation you HAVE ETERNAL SALVATION. If someone is telling you that you are not saved unless you remember the exact time you first believed and this bothers you, do what Bill Gothard suggested. Take a small sign, put today's date on it and put it in your yard. Any time someone tries to make you have doubts about your Salvation, just point them to that sign.
Remember, you did nothing to earn your Salvation. It was a gift from God. So you can't do anything that will make you loose it. Jesus is the author and finisher of both our faith and our salvation.
One last thing. Just because you can't lose your salvation once you posses it, does NOT mean you can just sin and sin. Ignore the rules God gives us to live by at your own risk. God will discipline us if we sin.
(Hebrews 12:5-11) "And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him: For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth. If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not? But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons.
Furthermore we have had fathers of our flesh which corrected us, and we gave them reverence: shall we not much rather be in subjection unto the Father of spirits, and live? For they verily for a few days chastened us after their own pleasure; but he for our profit, that we might be partakers of his holiness. Now no chastening for the present seemeth to be joyous, but grievous: nevertheless afterward it yieldeth the peaceable fruit of righteousness unto them which are exercised thereby"
Remember too, if we sin there is a cure!
"This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all. If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 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." (1John 1:5-10)