Following up on my partial response. So far I said:
"At the outset, His free will was unlimited. However, His desires are now holy, by His own design. He labored at least 13 billion years to maximize His zeal for holiness. At this point He is incapable of unjust or unkind behavior."
Those words of mine summarize the 1st prong in God's two-pronged solution to remaining holy. No temptation is strong enough to move Him to sin.
But that first prong isn't enough because the very nature of free will is such that, over time, His zeal could slowly decline. What's to guarantee that doesn't happen? And if such can happen, how can He guarantee us everlasting safety? Enter the 2nd prong.
Before we proceed any further, I want you to do me a favor. Turn off your immune system. Shut it down. Ok, is it off yet? What's taking you so long? You can't shut it off, can you?
My belief is that God created within Himself an Immune System of sorts that He cannot shut down, to guarantee His everlasting holiness and our eternal safety. Let's see how it works.
Passions - including the zeal for holiness - can be physically imposed. Take for instance how, at puberty, our hormones tend to impassion us toward the opposite sex. As Christians, we all hold to a doctrine called sanctification - the doctrine that if we seek God
properly, He will gradually increase our holiness by impassioning us toward righteousness. (The word in bold reflects my conviction that the church has been going about it all wrong for the last 2,000 years - long story).
Specifically it is the third Person of the Trinity - the Holy Spirit - who does this work. (That title is yet another error in traditional church doctrine, the correct tile is the Holy Wind/Breath).
But just as it is His job to MAKE us holy, it is also His job to KEEP God holy. What I'm saying is that the Third Person functions as the divine Immune System providing immunity to sin. God is not currently capable of sin, as I explained, but He could gradually decline in zeal. The Immune System monitors every particle of the Godhead, including those of the Immune System itself, such that if any particle declines even ONE IOTA in holy zeal, the REMAINDER of the Immune System will rush to immediately 'zap' that 'germ' (i.e. rekindle its holy desires).
Why can't God turn it off by free will? Theoretically He can, but only
gradually, and therefore it will never happen. Why not? Recall that He is so holy that He cannot sin immediately, first He would have to
gradually decline in holy zeal as to recover His original freedom. In the same way, His zeal to keep the Immune System running at top speed is so maximized that He can only
gradually decline in the zeal to keep it turned on. He can't just shut it off on a quick decision. The problem is that, if any particle in the Immune System loses even one iota of zeal to sustain the Immune System, the REMAINDER of the Immune System will instantly 'zap' this 'germ'.
As a result, it CANNOT be shut down. He CANNOT turn it off, neither now nor in the future.