All Bible quotations are from the King James Version.
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This is an attempt to make a logical proof, so if we can know it’s true, we must be able to know logical proofs work. If God has shown us that what we call “logic” can’t be broken, proofs only from “logic” without flaws and other true information are right. Here’s why I think I can know God agrees that logic can’t be broken.
The law of noncontradiction is one of the “laws of logic.” According to it, two different statements cannot be true in the same time and in the same sense. For example, a box cannot both be completely green and completely red.
The statement “it is true that there is no truth” is a statement that contradicts itself.
The law shows that truth has to exist and cannot not exist. It works in regard to the very existence of truth.
Since the very existence of truth abides by the law of noncontradiction, anything that exists as true abides by it.
For someone to understand this proof, they have to be able to know what words mean and to see what time and sense they’re in.
Therefore, someone who understands this proof can see when something breaks the law of noncontradiction.
The Greek word for “Word” in John 1:1 is logos. In that verse, logos means “Being with logic”(9). The law of noncontradiction cannot be broken because it comes from God’s way of being the Logos (Being with logic).
This is the order of points that arrive at “once saved, always saved.”
God is a perfect Being.
God considers genuinely anyone of the entire world having everlasting life as the utmost and uncompromisable importance.
Therefore, God must address anyone of the entire world as of the utmost and uncompromisable importance.
Just between them and God, God can keep a Christian saved genuinely through their free will.
Perfect creation must perfectly align with the values of God, Who is its Creator. He can be His perfect Self in the reality He has made.
In the reality that believers pass through which God hasn’t made (a sinful reality), He can ensure they genuinely choose to be whatever way(s) they must be to have everlasting life.
Since all of creation and God for all time has been addressed, this logic shows that someone who ever was saved cannot become unsaved.
God gave up His life so you could have everlasting life (John 1:1; 3:16). This life is “in his Son” (1 John 5:11): it is His life. That means, to God, it is just as important that you have everlasting life as it is that He has everlasting life.
To God, any person He died for is as valuable as the Greatest Being. No one can be more valuable than the Greatest Being, and all He died for are as valuable. We are all equally of the utmost value.
If God considered it worth it to let someone or something get in the way of your everlasting life, He would be valuing it over what has the utmost value. That would make you of lesser value when He said you aren’t. That would be a contradiction.
If someone is sane, loves to hangout with their friends, and is happy with their life, they will choose going to a party over driving off a cliff. Only a crazy person can do something crazy. If they come to their senses, they’ll stop (cf. 1 Cor. 15:34).
If someone is truly willing to be saved, then they ultimately value Heaven over being a sinful way. They are willing to have their thoughts and feelings align enough with God to lose their sinful nature and go to Heaven.
Reality with God is objectively more desirable than not, because God is objectively the most desirable existence. If they really thought He was, their thoughts and feelings would align with the truth.
If someone rightfully recognizes their options in both thought and feeling, and chooses from that, their will is free.
God is perfect, so everything of His personal experience is perfect, so He “cannot be tempted” (James 1:13; cf. 2 Tim. 2:13; Heb. 6:18). We, who are made to be like God (Gen. 1:27; James 3:9; cf. Eph. 4:24), are most like Him when we aren’t prone to temptation. It’s no wonder God said “so shall we ever be with the Lord” (1 Thess. 4:17).
In this view, every Christian told God out of their own free will that they wanted to live forever (“everlasting life”). God said, "Certainly. I’ll do what it takes for that.”
If there were to be even only one way a Christian had to act to be saved, that would be the way that had God, and so would be the most objectively desirable. If God aligned a saved person’s thoughts and feelings for that, it would agree with objective truth and their will to have everlasting life.
The above explanation for “once saved, always saved” comes from clear and uncontroversial Bible interpretation and logic. It takes logic to interpret anything written by the logical Author of the Bible.
Because we are made to be like God and God cannot be tempted and is perfect,
in Heaven, we’ll be the most like Him we will ever be! It’s no wonder He seals His children in Himself (Eph. 1:13-14)!
The very thing it takes to interpret anything of the Bible at all shows that we should be “confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ” (Phil. 1:6).