You cannot lose your salvation as Jesus' sacrifice on the cross was perfect.
When Christ drank the cup of God’s wrath He drank it to the dregs. There is no condemnation for those in Christ Jesus and that means on the final day and every day until then. God is never judging you for your sin if you are in Christ. That does not mean we are free to sin, as sin is still deadly and has consequences.
Christ came to redeem from sin. He took the punishment of every believer. God cannot judge His people in any way because his people have already been judged in Jesus. God no longer looks at Christians as sinners, but He sees His Son and His precious blood.
So either you have been judged in Christ, and because Christ was without sin the punishment of death was invalid...or you will be judged for your sins on judgement day.
John 6 39 And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day.
Teaching assurance of salvation is a good thing, we should be taught that we can be assured of the grace, promises, and kindness of God which is in Christ, and that God saving us is a precious treasure we can have total and complete confidence in.
My problem, however, with the false doctrine known informally as "once saved always saved", is that it not only ignores the seriousness of the warnings contained in Scripture concerning apostasy and the dangers of shipwrecking our faith. But, quite paradoxically, "once saved always saved" very easily undermines the very idea of assurance, undermining the confidence we can have in the Gospel.
If a brother or sister approaches us with worry, doubt, and despair over their salvation, we should point them to the promises of God in the Gospel. Now the trouble arises when we stop pointing to God's promises, and instead try to find an answer within ourselves; and this happens very easily when, as tragically happens, a Christian departs from the faith and ceases to be a Christian, whether for a season or--most tragically--even until the end of their mortal life. For those who insist that it is impossible to abandon our salvation, there are only two options that I am aware of for how to deal with that situation:
1) A Christian who has departed from the faith is merely in a state of backsliding, their salvation remains secure even though they are at present declaring themselves a non-beleiver.
2) More often, however, the answer is that this individual was never actually a Christian in the first place, they were a false believer, they never had faith, and thus they never had salvation anyway.
There are certainly problems with the first answer, but worse is the second answer by far. In the second answer all assurance is eradicated, and all confidence in the Gospel is thrown away. If a brother comes to me with doubts and questions, I should, as already said, be able to point them to the promises of the Gospel: I can point them to what is declared in the Scriptures that none shall be plucked from the hand of Christ, that there is no power that can separate us from the love of God that is for us in Christ, I can point to one's baptism and the promises of God attached there, to all the wonderful promises of God in the Gospel.
Now what is done with those promises if we take two professing Christians, and all things being equal, their lives have been identical as it pertains to the life of Christian faith. Then, circumstances arrive, and we have one brother who remains a professing Christian, but the other brother denounces his faith, and departs as an unbeliever.
If I, or anyone, were to say, the second brother was never a brother; they never believed, etc. And the first brother comes to me, and understandably upset and experiencing the pain of seeing the other brother leave and I tell him the second brother was never really saved; the first may then seek comfort--"how can I know that I'm saved?"--having shared the same experiences, growing up in faith, baptized, hearing the same Scriptures, fellowshipping together, living as Christian brothers--what am I to say? That the reason why the first brother is actually saved is because of something he did differently? That his faith was genuine but the second brother's faith was false? This raises the deeply existential dread: How can I know whether my faith is genuine or false? Am I a true believer or a false believer?
What must I do in order to be a true believer rather than a false one?
Because how can I point to the promise of God as objectively true and assured if the one, hearing and receiving the same promises from God, never actually had God's promise? If his faith was false rather than genuine? Then how can we be assured by God's promise? We are forced to instead look inward, to ourselves, or to our works, or some other thing other than the promises of God.
And the moment we do that there will not, and cannot, be assurance. There could be pride or vanity, but that is not assurance that is self-righteousness. And all that is really left is unconsolable despair.
Our consolation, our comfort, our assurance, our confidence must be in the promises of God and nothing else.
The brother who throws away his faith is not guilty of never having had faith, but of throwing it away; it is not that the word of God is untrustworthy, it is not that the promises of God fail; it is that apart from Christ we are without hope. And our salvation exist in Jesus, and our confidence and assurance is always in Christ, and the promises of God are inviolate, there is nothing that can separate us from the love of God, there is no one that can pluck us from the Savior's hand. To trust in Christ is to be the child of God.
That is our confidence, that is our assurance.
Once saved always saved can lead to despair, it can lead to pride, but it can not lead to the comfort, consolation, confidence, and assurance of the Holy Gospel.
-CryptoLutheran