The Bible tells of salvation through family trees, explaining events that were to come and which then came. Leaving out the parts before this (which were blessing to generation, blessing to generation, etc.), I can focus on Jacob, who had 12 sons. His sons became the 12 tribes of Israel, each tribe being named after one of the sons. When he was about to die, Jacob told his son Judah,
“Judah, your brothers shall praise you;—Genesis 49:8-12
your hand shall be on the neck of your enemies;
your father’s sons shall bow down before you.
Judah is a lion’s cub;
from the prey, my son, you have gone up.
He stooped down; he crouched as a lion
and as a lioness; who dares rouse him?
The scepter shall not depart from Judah,
nor the ruler’s staff from between his feet,
until tribute comes to him;
and to him shall be the obedience of the peoples.
Binding his foal to the vine
and his donkey’s colt to the choice vine,
he has washed his garments in wine
and his vesture in the blood of grapes.
His eyes are darker than wine,
and his teeth whiter than milk.
The scepter is talking about a king’s staff. In Hebrew, it means “ruler’s staff” and uses a root word that means cutting or inscribing, the verb used to mean inscribing of laws. Jews would have understood this prophecy to mean that a future king would come from the line of Judah. Unusually, the prophecy also said that the king would rule over nations, not just one nation. This was not typically a Jewish thought, as they were an exclusive lot in that regard.
The first Israelite ruler from Judah was King David. As he was dying, the prophet Nathan told him,
“Moreover, the LORD declares to you that the LORD will make you a house. When your days are fulfilled and you lie down with your fathers, I will raise up your offspring after you, who shall come from your body, and I will establish his kingdom. He shall build a house for my name, and I will establish the throne of his kingdom forever.”—2 Samuel 7:11-13
God didn’t mean for David to be a forever kind, as he remarked that David was going to die. Still, he repeated the word “forever” when talking about David’s legacy. The prophecy wasn’t about Solomon, who had a temporary rule, or Zedekiah, who was taken captive and was in fact the last of the lineage of David to be a king. What was it about?
Luke said that an angel spoke to Mary hundreds of years later. Mary was a downstream scion of David, and she was told,
“Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”—Luke 1:30-33
Jesus was the one designated to rule over Jacob’s descendants forever, with an endless kingdom. This ended up being offensive to many Jews, who were hoping for an earthly strongman. The author of Hebrews explained that the prophecy of Psalm 110:1-7 (hover/click for text) was fulfilled by Jesus:
“But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, waiting from that time until his enemies should be made a footstool for his feet. For by a single offering he has perfected for all time those who are being sanctified.”—Hebrews 10:12-14
God made a promise to Abraham that through him, all nations would be blessed. Downstream of that promise, David became a king, and things were made clearer. The king of the nation forever was no earthly king giving blessings of wealth or material things, but Jesus Himself, blessing all nations forever by bringing salvation to the world.