His Faith.
What Faith is Paul speaking of?
... Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone;
For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them. But the righteousness which is of faith speaketh on this wise, Say not in thine heart, Who shall ascend into heaven? (that is, to bring Christ down from above; Or, Who shall descend into the deep? (that is, to bring up Christ again from the dead.) But what saith it? The word (through Christ) is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach; For Christ (through which the word is in the heart and mouth) is the end of the law (on tables of stone and parchment) for righteousness to every one that believeth. (Rom 9:31-32; 10:4-8)
As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed.
What if they choose not to believe, what if we refuse the faith that he imparts. To live in unbelief. Outside of the guidance of The word in our heart and mouth?
Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: For if God spared not the natural branches, take heed lest he also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
(Rom 11:20-22 KJV)
So if Faith is from Christ through the Word is in the heart and Mouth What is unbelief? Here is an excerpt from the BDAG in regards to the Greek word translated unbelief. Bold emphasis mine
ἀπιστία, ας, ἡ (s. ἀπιστέω; Hes., Hdt. et al.; LXX, Philo, Joseph.; Just.; Tat. 32, 2; Ath. R. 60, 15 al.; s. Mayser 11f, 130)
unwillingness to commit oneself to another or respond positively to the other’s words or actions, lack of belief, unbelief ....
lack of commitment to a relationship or pledge, unfaithfulness (X., An. 3, 2, 4 et al.; UPZ 18, 5 [163 B.C.]; Wsd 14:25; Philo, Spec. Leg. 2, 8, Decal. 172; Jos., Ant. 14, 349) Ro 3:3 (JGriffiths, ET 53, ’41, 118).—M-M. TW.
So unbelief is A lack of commitment, an unwillingness to commit oneself to another or respond positively to the other’s words, THE WORD IN THE HEART AND MOUTH through Christ. And because of this they were broken off, and WE standest by faith. So Be not highminded, but fear: For if God spared not the natural branches, take heed lest he also spare not US. Behold therefore the goodness and severity of God: on them which fell, severity; but toward US, goodness, if WE continue in his goodness: otherwise WE also shalt be cut off.