This verse is talking about the nation of Israel, part of them have become calloused or hardened.
Looking back a few verses in Romans 11, it shows that there were a few out of the nation of Israel that believed the evangel . This remanent, or little flock was totally a choice of grace on God's part.
so then also, in the present time, there has been a remnant according to the election of grace. 6And if by grace, it is no longer from works; otherwise grace no longer would be grace.c
7What then? What Israel is seeking, this it has not obtained, but the elect obtained it. And the rest were hardened, 8as it has been written:
“God gave them a spirit of stupor,
eyes not to see,
and ears not to hear,
unto this very day.”
This callusing or hardening of Israel is only temporary.
for I do not wish you to be ignorant, brethren, of this secret -- that ye may not be wise in your own conceits -- that hardness in part to Israel hath happened till the fulness of the nations may come in.
The word hardness is described:
4457. pórósis ►
Thayer's Greek Lexicon
STRONGS NT 4457: πώρωσις
πώρωσις, πωρωσεως, ἡ (πωρόω, which see), properly, the covering with a callus; tropically, obtuseness of mental discernment, dulled perception: γέγονε τίνι, the mind of one has been blunted (R. V. a hardening hath befallen), Romans 11:25; τῆς καρδίας (hardening of heart), of stubbornness
Here are the three times the hardness of heart is used.
Mark 3:5 N-DFS
GRK: ἐπὶ τῇ πωρώσει τῆς καρδίας
NAS: grieved at their hardness of heart,
KJV: for the hardness of their
INT: at the hardness of the heart
Romans 11:25 N-NFS
GRK: φρόνιμοι ὅτι πώρωσις ἀπὸ μέρους
NAS: that a partial hardening has happened
KJV: that blindness in
INT: wise that hardness in part
Ephesians 4:18 N-AFS
GRK: διὰ τὴν πώρωσιν τῆς καρδίας
NAS: that is in them, because of the hardness of their heart;
KJV: because of the blindness of their
INT: on account of the hardness of the heart
A very striking statement is then made in the following verse. The hardness or callousness of Israel's heart will vanish and All of Israel will be saved.
And so all Israel shall be saved, according as it hath been written, ‘There shall come forth out of Sion he who is delivering, and he shall turn away impiety from Jacob,
And this to them [is] the covenant from Me, when I may take away their sins.' Romans 11:26
We know from Israel's history, that, even though they had the law of God, they still had the law of sin and death working in them, just like the rest of the nations. The elected believers by grace in Israel are the the fruit of the whole..
"When the nation of Israel is saved as a whole, regains is proper place the whole nation, from the least to greatest, will know God. And they will become the light of the world, as they were always intended to be. " Concordant Commentary.
It will be life out of the dead for that nation.
If they're casting away is the conciliation of the world, what will their taking back be, if not life from among the dead?
Romans 11:15