Doctrine of Immutability
I so call this doctrine a "doctrine of immutability" because it is our state that is unchanging. Our state being our status before God, on the basis of His Son. It is a present being, as to what the Lord calls us. These shall be facts immutable, irrevocable, and binding, as the self-evident truths of God's word. We will not be shaken because our foundation is sure and what winds, of doctrine, that fly by shall simply brush off us and not remain. As God's word says, what fellowship does darkness have with light? So let us illuminate the truth as darkness retreats and deception crumbles.
Often a case is made, to the detriment of our peace, using multiple verses that presumably reveal the possibility of our salvation being lost. This they do in contrast to what might be called "over arching truths." Comprehensive truths, that once seen cannot be unseen. This is not a verse or two, or even multiple verses to formulate a doctrine (with the bias of presupposition). This is going to be a stating of facts, clearly revealed in God's word. Not a list of verses confirming a position (as both sides are well equipped). No, we shall set the stage, see the intended consequences of Christ's actions and roles, and then see the big picture. The will of the Father.
Mankind Rebels
Where better to start, than the beginning? We can only speculate in our perceptions of God revealed in His word as to the thoughts that went through His mind before speaking the world into existence. His foreknowledge and sovereignty permitting the existence of all that the universe encompasses, yet out of His good pleasure He endures it as Christ bore the cross with the joy set before Him. Merciful, indeed. Patient, of course. His long-suffering being the backbone to our repentance, as His desire is that none perish. That all would come to repentance.
What circumstance has man placed himself in that a holy and righteous God would call him to repentance? Condemnation, the path of the unrighteous for those outside of Christ. Adam and Eve being the catalyst of humanity's alienation from God through disobedience, and sin not being allowed in the presence of God, Almighty. A barrier between God and man. Sin's penalty is death, and God's judgement is righteous. What shall God do with a people incapable of redemption, in and through themselves? Will He let them suffer the consequence of their actions? He could have, but instead He chose mercy out of His abundant love for His creation. He sent His Son, and this is where we begin to see our status before God unfolding.
Reconciliation
God so formulated a plan of reconciliation that would tear down the barrier between man and God that nothing could separate us from His love. Jesus went to the cross, died our death (atonement), and resurrected unto our justification before God. This was through His bloodshed, as it says in His word that "without the shedding of blood, there is no remission." Yet even greater and followed it is stated, "...where remission of these (sin) is, there is no more offering for sin." Our sins have been remitted, canceled a debt or punishment, because of the Lamb of God.
Through this act the barrier between the Lord and His creation was torn (symbolized in the veil being split in two). Sin, all sin, being remitted through the blood of Christ. Total forgiveness, redemption ours. Sin can no more alienate a believer from God than Jesus will go to the cross again. It is impossible, because He did one sacrifice for all time, for all sin. Not being like the priests of old having to do daily and yearly sacrifices, He being eternal and risen, was made an eternally sufficient sacrifice for all sin. His act then still applies today, how else would the lost be saved? Jesus' blood has granted the remission of sin (in its entirety).
Justification
We, at this point, now see one aspect of our status before God that is unchanging. Whilst this is no license to sin, sin no longer condemns. We stand justified before God on the basis of His Son. Sin as destructive as it is, and replete with natural consequences, does not lead to eternal condemnation as a believer because Jesus, through His bloodshed, has granted us the remission of sin. We are justified before God, fact. This is one truth that is self-evident, irrevocable, immutable, and binding. Is there yet more?
High Priest
I ought to further elaborate on Jesus' role here, as our High Priest. This too shall be an overarching truth that maintains our status before the Lord. Yet another truth that cannot be unseen, erupting praise. What role and function does the high priest serve? A mediator before God and man, Jesus interceding on our behalf. Our High Priest after the order of Melchizedek, with a priesthood that is eternal. Where as the priests of old died and therefore so did their service, Jesus lives on (resurrected) able to intercede on our behalf forever. For this reason He is able to save us completely or to the uttermost.
There is no condemnation upon us, for we are in Christ and the Lord as our faithful High Priest makes intercession for us. This priest, our Lord, doesn't make continual sacrifices as others did. For He is the very sacrifice, and this being so, if His blood didn't remit all sin He would've had to suffer since the foundation of the world (since He is the sacrifice for sin). Yet, His blood is sufficient, eternal, and His sacrifice the propitiation for the sins of the world. Since the role of the high priest is, essentially, to make peace with God, appeasing His justice we have this in Jesus always (as He lives on forever, as does His priesthood; including His sacrifice and bloodshed).
Here we yet find another aspect of immutability. Our place of justification before God based upon the blood of Christ, unchanging, as the Lord our High Priest intercedes forever upon our behalf. Our Mediator, our Advocate with the Father. Our position and status secure, found in Christ. This is too self-evident as we comprehend the role of the high priest and Jesus' advocacy on our behalf.