How much sin does it take to lose your salvation... is that like how many licks does it take to get to the center of a tootsie pop? The world may never know
Seriously... Most people who have discussed this topic on the forums here know that I believe it is possible to turn away from God and become apostate. I don't like the phrase "lose your salvation" because it tends to make people think you can lose your salvation like you would your keys.. oops.. now where did I put that salvation again!
Salvation is not lost because you sinned one too many times, or you did a sin that society considers to be really bad... Salvation is lost, in the opposite of the way it is gained. It is gained by receiving the grace of God and submitting to its work in you. It is lost by rejecting the grace of God and refusing to submit to its work in you.
Now.. what can lead to that situation? The bible is clear that deception can cause people to fall "the Spirit speaks expressly that in latter days many shall depart from the faith, (why?) giving heed to seducing spirits..." allowing themselves to be lead astray by the doctrines of devils and breaking their communion with God.
Hardening your heart against God is also something that can lead people to reject him and his grace. This could be from anger or bitterness, but also simply out of a love of the world, a continued refusal to repent which eventually leads to a person chosing the world over God.
James 1:15 "when lust has concieved it brings forth sin, when sin is finished, it brings death." We can never lose salvation by sinning one too many times (remember that the righteous man is he that falls seven times and seven times gets up), but allowing sin to fester and breed and refusing to repent can put you in danger.
2nd Chronicles 15:2 "... the Lord is with you while you are with him, if you seek him he will be found by you, if you leave him, he will leave you."
The chief danger for us in all cases is that we grow lax and unconcerned.. that opens us up to deception, it allows our heart to cool, and our desire for righteousness and truth to wane...
1 Tim. 1:19 "keeping faith and good conscience, which some have rejected and their faith has been shipwrecked."
Heb. 2:1 "Therefore we must give the more earnest heed to the things we have heard, lest we drift away."
2:2 "For if the word spoken through angels proved steadfast, and every transgression and disobedience received a just reward, "
2:3 "how shall we escape if we neglect so great a salvation, which at the first began to be spoken by the Lord, and was confirmed to us by those who heard Him, "
In my opinion, thinking that it is impossible for you to fall is probably the single most dangerous idea a christian can have.
1 Cor. 10:12 "Wherefore let him that thinketh he standeth take heed lest he fall. "