Non-abiding belief, so not genuine belief?
When the person was believing > 50% so as to act commensurate with trust in Christ, they were abiding and they had abiding faith.
When the person stops believing > 50% so that they cease acting commensurate with trust in Christ, they have ceased to abide, and have therefore ceased to have abiding faith.
What do you make of Romans 12:1-3
Romans 12:1–3 (NKJV): 12 I beseech you therefore, brethren, by the mercies of God, that you present your bodies a living sacrifice, holy, acceptable to God, which is your reasonable service. 2 And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
3 For I say, through the grace given to me, to everyone who is among you, not to think of himself more highly than he ought to think, but to think soberly,
as God deals (aorist active indicative) to each one a measure (anarthrous accusative) of faith (anarthrous genitive).
Λέγω γὰρ διὰ τῆς χάριτος τῆς δοθείσης μοι παντὶ τῷ ὄντι ἐν ὑμῖν μὴ ὑπερφρονεῖν παρ᾽ ὃ δεῖ φρονεῖν ἀλλὰ φρονεῖν εἰς τὸ σωφρονεῖν
ἑκάστῳ ὡς ὁ θεὸς ἐμέρισεν μέτρον πίστεως
I see this as saying that we each have varying degrees of faith mixed with unbelief/doubt that God gives, not by pouring some commodity called faith into us, but by doing and saying things to and around us that inspire a degree of faith, and we should not overstate the degree of our faith, because God desires that we walk in truth, not mixed with any pretentiousness/hypocrisy, for a little leaven threatens to leaven the whole lump. We see examples of such pretentious hyperfaith from faith preachers who present themselves as men of great faith in God and Christ trusting 100% in Them, but who have developed such a desire for and dependency on and trust in their riches, that their faith in Christ is largely unfruitful, if one regards fruit as the fruit of the Spirit.