Faith is the subject. This Faith in which we speak comes from Christ; the Word in our hearts and mouths. Israel was focusing on the works of the Law but their hearts were never of GOD. They stumbled at the stumblingstone having never had Faith that is of GOD through HIS Christ. This Faith has always been available to them that was Paul through the Spirit was sharing when he paraphrased Deuteronomy 30:12-14. But they never received it. They stumbled at the stumbling stone
What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith
(For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another; )
(Rom 2:14-15 KJV)
. But Israel, which followed after the law of righteousness (
that which is not of faith), hath not attained to the law of righteousness. Wherefore? Because they sought it not by faith (
they would not receive the promise of GOD; HIS Word; Christ in the Heart, mouth and mind) , but as it were by the works of the law (
by what they were doing not by what they could be through GOD'S indwelling changing power. The circumcision made without hands)). For they stumbled at that stumblingstone (; As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed. Brethren, my heart's desire and prayer to God for Israel is, that they might be saved. For I bear them record that they have a zeal of God, but not according to knowledge. For they being ignorant of God's righteousness (
which is through a heart of faith), and going about to establish their own righteousness (
obedience from the mind and not from the heart), have not submitted themselves unto the righteousness of God. For Christ (
the word In the heart and mouth) is the end of the law (
that written with ink and on tables of stone)for righteousness to every one that believeth. 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 (
the word; The Commandments contained in the book of the Law that pertain to righteousness) down from above
Or, Who shall descend into the deep? (
that is,, to bring up Christ (
the word; The Commandments contained in the book of the Law that pertain to righteousness) again from the dead.) But what saith it? The word is nigh thee, even in thy mouth, and in thy heart:
that is, the word of faith, which we preach; That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
For with the heart man believeth into righteousness; and with the mouth confession is made unto salvation.
(Rom 9:30-10:14)
"That is" in the above text in each instance connects to what is before not what is after. Here is the Strong penned.
G5123 τουτέστι toutesti (tout-es'-tee) v.
1. that is
[contraction for G5124 and G2076]
KJV: that is (to say)
In other words when Paul wrote, "That is the faith in which we preach" He was referring to what was previously stated. The word; Christ being nigh unto us. In our hearts and mouths is the Faith in which we preach and this is the Faith that Israel in general was missing but had available to them since Moses preach it in Deut. 30:12-14. The circumcision made without hands.