Definition of APOSTASY http://www.merriam-webster.com/dictionary/apostasy
1: renunciation of a religious faith
Blue Letter Bible: to fall away (from the true faith): from worship of Jehovah
I believe Salvation can't be lost by sinning, but can be lost by Apostasy (i.e. renouncing Christ). I have arrived at this through my personal study of Hebrews, especially Chapter 6:1-12 and also Chapter 10. Here we have Hebrew Christians that were being persecuted for their faith in Christ and as a result were considering a return to Judaism. They are being warned (6:4-6) in the strongest terms that if they "fall away" (renounce Christ) and return to Judaism, there is no coming back again. Verse 4 calls them "partakers of the Holy Spirit", which to me settles the issue that they are saved, born again Christians, in spite of protests to the contrary from many commentators. I do believe in Eternal Security, but with Apostasy being the lone exception.
Hebrews 6:1-12 NKJV "Therefore, leaving the discussion of the elementary principles of Christ, let us go on to perfection, not laying again the foundation of repentance from dead works and of faith toward God, of the doctrine of baptisms, of laying on of hands, of resurrection of the dead, and of eternal judgment. And this we will do if God permits. 4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame. For the earth which drinks in the rain that often comes upon it, and bears herbs useful for those by whom it is cultivated, receives blessing from God; but if it bears thorns and briers, it is rejected and near to being cursed, whose end is to be burned. But, beloved, we are confident of better things concerning you, yes, things that accompany salvation, though we speak in this manner. For God is not unjust to forget your work and labor of love which you have shown toward His name, in that you have ministered to the saints, and do minister. And we desire that each one of you show the same diligence to the full assurance of hope until the end, that you do not become sluggish, but imitate those who through faith and patience inherit the promises."
Now look at Hebrews Chapter 10 (NKJV):
1 For the law, having a shadow of the good things to come, and not the very image of the things, can never with these same sacrifices, which they offer continually year by year, make those who approach perfect. For then would they not have ceased to be offered? For the worshipers, once purified, would have had no more consciousness of sins. But in those sacrifices there is a reminder of sins every year. For it is not possible that the blood of bulls and goats could take away sins. Therefore, when He came into the world, He said: "Sacrifice and offering You did not desire, But a body You have prepared for Me. In burnt offerings and sacrifices for sin You had no pleasure. Then I said, 'Behold, I have come— In the volume of the book it is written of Me— To do Your will, O God.'" Previously saying, "Sacrifice and offering, burnt offerings, and offerings for sin You did not desire, nor had pleasure in them" (which are offered according to the law), then He said, "Behold, I have come to do Your will, O God." He takes away the first that He may establish the second. By that will we have been sanctified through the offering of the body of Jesus Christ once for all. And every priest stands ministering daily and offering repeatedly the same sacrifices, which can never take away sins. But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, from that time waiting till His enemies are made His footstool. For by one offering He has perfected forever those who are being sanctified. But the Holy Spirit also witnesses to us; for after He had said before, "This is the covenant that I will make with them after those days, says the LORD: I will put My laws into their hearts, and in their minds I will write them," then He adds, "Their sins and their lawless deeds I will remember no more." Now where there is remission of these, there is no longer an offering for sin. Therefore, brethren, having boldness to enter the Holiest by the blood of Jesus, by a new and living way which He consecrated for us, through the veil, that is, His flesh, and having a High Priest over the house of God, let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for He who promised is faithful. And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day (End) approaching. 26 For if we sin willfully (In context this willful sin is apostasy) after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses' law dies without mercy on the testimony of two or three witnesses. 29 Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot (Apostasy), counted the blood of the covenant by which he was sanctified a common thing (Apostasy), and insulted the Spirit of grace? (Apostasy). For we know Him who said, "Vengeance is Mine, I will repay," says the Lord. And again, "The LORD will judge His people." It is a fearful thing to fall into the hands of the living God. 32 But recall the former days in which, after you were illuminated, you endured a great struggle with sufferings: partly while you were made a spectacle both by reproaches and tribulations, and partly while you became companions of those who were so treated; for you had compassion on me (the Apostle Paul) in my chains, and joyfully accepted the plundering of your goods, knowing that you have a better and an enduring possession for yourselves in heaven (they're Saved). Therefore do not cast away your confidence (Faith), which has great reward. For you have need of endurance, so that after you have done the will of God, you may receive the promise: "For yet a little while, And He who is coming will come and will not tarry. 38 Now the just shall live by faith; But if anyone draws back, My soul has no pleasure in him." But we are not of those who draw back to perdition, but of those who believe to the saving of the soul.