Jeremiah 33:17-18
For this is what the Lord says: "David will never fail to have a man to sit on the throne of the house of Israel, nor will the priests, who are Levites, ever fail to have a man to stand before me continually to offer burnt offerings, to burn grain offerings and to present sacrifices."
When will this promise be fulfilled? Not in this day, today. Not in the forseeable future, naturally speaking. But when Jesus returns, He will sit on David's throne as the legal, regal King of Israel, and He will appoint the priests who He knows are Levites, to serve, during His 1000-year reign on this Earth.
Even so, come, Lord Jesus!
