Read the rest of the story in Ro 9:27, 11:4-5. . .and note the "if" in Ro 11:23.
Romans 9:27-28
27 Esaias also crieth concerning Israel, Though the number of the
children of Israel be as the sand of the sea, a remnant shall be saved :
28 For he will finish the work, and cut it short in righteousness: because a short work will the Lord make upon the earth.
Isaiah 10:22-23
22 For though
thy people Israel be as the sand of the sea, yet a remnant of them shall return: the consumption decreed shall overflow with righteousness.
23 For the Lord GOD of hosts shall make a consumption, even determined, in the midst of all the land.
Romans 11:1-6
1 I say then , Hath God cast away his people? God forbid . For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin.
2
God hath not cast away his people which he foreknew. Wot ye not what the scripture saith of Elias? how he maketh intercession to God against Israel, saying,
3 Lord, they have killed thy prophets, and digged down thine altars; and I am left alone, and they seek my life.
4 But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
5 Even so then at this present time also there is a remnant according to the election of grace.
6 And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work.
1 Samuel 12:22 For
the LORD will not forsake his people for his great name's sake: because it hath pleased the LORD to make you his people.
"if"
1 Kings 19:10, 14, and 18
10 And he said, I have been very jealous for the LORD God of hosts: for the children of Israel have forsaken thy covenant, thrown down thine altars, and slain thy prophets with the sword; and I, even I only, am left ; and they seek my life, to take it away.
14 And he said, I have been very jealous for the LORD God of hosts: because the children of Israel have forsaken thy covenant, thrown down thine altars, and slain thy prophets with the sword; and I, even I only, am left; and they seek my life, to take it away.
18 Yet I have left me seven thousand in Israel, all the knees which have not bowed unto Baal, and every mouth which hath not kissed him.
Romans 11:23 23 And they also,
if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.
It doesn't appear that God has changed after the fulfillment of Christ. And you say........
The only such willful sin in the NT is unbelief.
Seems the same was also in the OT.