The Davidic covenant is an unconditional covenant between God and David. Unconditional meaning that there is nothing that David needs to do in order for God to keep His promises.
1 Chron 17:11-14
11 And it shall come to pass, when thy days be expired that thou must go to be with thy fathers, that I will raise up thy seed after thee, which shall be of thy sons; and I will establish his kingdom. 12 He shall build me an house, and I will stablish his throne for ever. 13 I will be his father, and he shall be my son: and I will not take my mercy away from him, as I took it from him that was before thee: 14 But I will settle him in mine house and in my kingdom for ever: and his throne shall be established for evermore.
Looking back at Israel’s history we can see that after Soloman, Israel was divided into the Northern Kindom and the Southern Kindom. The Northern kingdom was wiped out by the Assyrians as a punishment from God for their idolatry. Prior to being wiped out, many in the North who wanted to turn from idolatry went south. King Zedekiah was reigning in Judea at the time when Jeremiah the prophet was warning him about the same things that brought destruction to the north. The king ignored Jeremiah and Jerusalem was destroyed. The inhabitants were taken to Babylon for 70 years.
There has been no king in Israel since Zedekiah. When we look at this covenant you wonder how God can keep this promise? This is answered in the birth of Jesus:
31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. 32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
The Millennial reign at Jesus second coming is the fulfillment of the Davidic covenant. Paul connects this to the Jews that he is writing to in the epistle to the Hebrews. The Jews know the history of Israel and are well aware that they have no king. They know of God’s promise to David and were likely wondering how this can possibly be fulfilled.
8 But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom.
When the Jews realized that Jesus is the fulfillment of the Davidic covenant, they probably connected all the dots and took notice.
When it comes to the millennium where will we be, are we willing to put the effort in now to inherit (sanctification) or are we content to just enter. Time is our most valuable possession, lets put it to good use.