As Horatius Bonar says,“Take the prophecies regarding the incarnation of Christ. Before that event took place, there might be a controversy as to whether they were to be literally fulfilled or not. A Jew might have argued with much apparent force against a literal meaning, What! Is God to take upon Himself the form of a man? Is Jehovah to become an infant of days, nay, to be born of a creature, to be a man of sorrows and acquainted with grief, to die and be buried as men die and are buried? Impossible! The very idea is carnal beyond endurance. These prophecies cannot be interpreted in their literal sense; they must have some figurative, some spiritual meaning. So might a Jew have argued before Messiah came….(However the) fact, the glorious but stupendous fact, made known in the fullness of time, proved not only that the literal was the true sense of these prophecies regarding Messiah’s first coming, but also established this truth, that the literal interpretation and fulfillment may be the more truly spiritual of the two.”
Not all of the promises of God were fulfilled by Christ's first coming(or by the events of 70 a.d.). There are still promises yet to be fulfilled with the race of Israel, the descendants of Abraham, Isaac and Jacob. Israel was promised that the New Covenant would be applied to them as a people. They were promised that a descendant of David(Jesus Christ) would reign over them in the Promised Land, and that they would be greatly multiplied as a people(Davidic and Abrahamic Covenants).
Christ's first coming, His establishing a New Covenant, has guaranteed that Israel will inherit the promises to the Patriarchs(Abraham, Isaac and Jacob). The New Covenant validates God's promises to the Patriarchs, as it is the means by which those promises will be 'enacted' permanently upon their physical offspring. As Paul points out in Romans 15, "Christ became a servant to the circumcised to show God's truthfulness, in order to confirm the promises given to the patriarchs..."(Rom 15:8 ESV) Christ confirms those promises, and those promises will be fulfilled, but in the future, at a set time, which only the Father knows(Acts 1:6-7). Paul says, in Romans 11 that, in the future, all Israel shall be saved, just as was written.
So, at Christmas time we rejoice, not only in God's grace, but in His faithfulness to His promises. "Who is a God like you, pardoning iniquity and passing over transgression for the remnant of his inheritance? He does not retain his anger forever, because he delights in steadfast love. He will again have compassion on us; he will tread our iniquities underfoot. You will cast all our sins into the depths of the sea. You will show faithfulness to Jacob and steadfast love to Abraham, as you have sworn to our fathers from the days of old."(Mic 7:18-20 ESV)