A: This is a great question. The answer is, all Israel will be saved on that day...but by comparison with Israel's original numbers, those saved will be just a small remnant. Let me explain.
In Romans Paul is writing about the final condition of Israel after the Tribulation Period, which is referred to in Scripture as a "time of distress for Jacob." (Jeremiah 30:7). The latter half of the Tribulation will be a terrible time for the Jews, but when all hope seems lost, Jesus will return to the earth and deliver them from their enemies who are poised to annihilate the Israelites. When the Jews living on the earth see their Deliverer, they will recognize Him as the One "whom they have pierced" and, Zechariah tells us, they will "mourn for him as for an only child and weep bitterly over him..." (Zech. 12:10) Why is that? Because they will know for certain that He is Jesus—the One they had previously rejected.
The up side is that Israel will celebrate their deliverance with a national conversion to Christ on that day because God will give them a spirit of grace to call upon the Lord. Zechariah wrote:
"On that day there shall be a fountain opened for the house of David and the inhabitants of Jerusalem, to cleanse them from sin and uncleanness." Zechariah 13:1 (ESV)That is what Paul meant by the words, "all Israel will be saved." But Isaiah reminds us that although their original numbers were "as the sand of the sea," those saved will be but a small remnant by comparison.