You CAN forfeit your salvation.  You do NOT have the security of being ‘once saved always saved’, independent of your future choices and actions.  But people really misunderstand what this is saying!  It is not a matter of God turning His back on you the first time you sin, and abandoning you!!

‘If we believe not, [yet] he abideth faithful: he cannot deny himself.’  (2 Timothy 2:13)

Unintentional sin is also ‘covered’, if we are doing our best to be obedient to God and walk with Him.

‘[There is] therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.’  (Romans 8:1)

When we stumble and sin, the Holy Spirit shows us that we have sinned, and we repent.  (If we don’t, then that is the same as willfully sinning.)  Stumbling and repenting is covered too.

‘My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous’  (1 John 2:1)

(Notice this verse DOESN’T say ‘Jesus death COVERED you’ (past tense).  It says that we HAVE AN ADVOCATE with the Father.  Jesus WILL PLEAD our case before Him (future tense).)

But, here is how it works that one can lose their salvation . . .

WILLFUL sin (or willfully not repenting when the Holy Spirit shows you you’ve sinned) is what causes the problem.

‘For if we sin WILLFULLY after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins‘  (Hebrews 10:26)

The Bible tells us that he who sins is slave to sin.  Sin is deceitful.  It has a power of its own.  Once we sin, the power of sin over us usually persuades us to sin more and more.  The only thing to counterbalance that is the Holy Spirit.  But when a Christian willfully sins, they are ALREADY ignoring the Holy Spirit as it works in their heart and tries to persuade them not to sin.

The Holy Spirit, in a Christian’s life, guides and empowers them to walk in righteousness.  The Holy Spirit is God.  When we resist and disobey the Holy Spirit, we are resisting and disobeying God.  Repeated sin . . . repeatedly ‘quenching’ the Holy Spirit . . . will lead to hardness of the heart.  If that heart is not broken, and the person doesn’t repent, eventually their heart will become hardened to the point that the Holy Spirit can no longer reach them.  (Their conscience ‘seared with a hot iron’.)  We know that . . .

‘No man can come to me, except the Father which hath sent me draw him . . . ‘  (Jesus, John 6:44)

Once our heart becomes hardened to where the Holy Spirit cannot reach us, we can no longer experience godly sorrow over our sins, no longer be brought to repentance, no longer be reconciled to God!

In all of this, God remains blameless.  Throughout all this, the Holy Spirit pleads with us and shows us our sinfulness.  Meanwhile God, our loving Father, chastens us by bringing difficult circumstances into our lives, to bring us back to our senses.

‘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.’  (Hebrews 12:6-8)

The fault never lies with God.  The problem is the power of sin over those who practice it, and the hardening of the heart by the lack of repentance on our part.

But after thy HARDNESS and IMPENITENT HEART treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God’  (Romans 2:5)

If you choose to willfully sin, resist the Holy Spirit, and spurn all God’s attempts to turn you back to righteousness, you cannot fall back on the Scripture that says:

‘And I give unto them eternal life; and they shall never perish, neither shall any [man] pluck them out of my hand. My Father, which gave [them] me, is greater than all; and no [man] is able to pluck [them] out of my Father’s hand.’  (John 10:28-29)

You can’t claim that someone ‘plucked you out of God’s hand’ when it was YOU who made the choice to willfully sin!

Additional Scriptures:

For [it is] impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put [him] to an open shame.

For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God:  But that which beareth thorns and briers [is] rejected, and [is] nigh unto cursing; whose end [is] to be burned.‘  (Hebrews 6:4-8)

‘For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.

He that despised Moses’ law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?

For we know him that hath said, Vengeance [belongeth] unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.  It is] a fearful thing to fall into the hands of the living God.’  (Hebrews 10:26-31)

‘For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning.  For it had been better for them not to have known the way of righteousness, than, after they have known [it], to turn from the holy commandment delivered unto them.’  (2 Peter 2:21)