Now therefore thus you shall say to my servant David: Thus says the Lord of hosts: I took you from the pasture, from following the sheep, to be ruler over my people Israel; and I have been with you wherever you went, and have cut off all your enemies before you; and I will make for you a name, like the name of the great ones of the earth. I will appoint a place for my people Israel, and will plant them, so that they may live in their own place, and be disturbed no more; and evildoers shall wear them down no more, as they did formerly, from the time that I appointed judges over my people Israel; and I will subdue all your enemies. Moreover I declare to you that the Lord will build you a house. When your days are fulfilled to go to be with your ancestors, I will raise up your offspring after you, one of your own sons, and I will establish his kingdom. He shall build a house for me, and I will establish his throne forever. I will be a father to him, and he shall be a son to me. I will not take my steadfast love from him, as I took it from him who was before you, but I will confirm him in my house and in my kingdom forever, and his throne shall be established forever. (1 Chronicles 17:7-14, NRSV)
Now therefore thus shalt thou say unto my servant David, Thus saith the Lord of hosts, I took thee from the sheepcote, even from following the sheep, that thou shouldest be ruler over my people Israel: And I have been with thee whithersoever thou hast walked, and have cut off all thine enemies from before thee, and have made thee a name like the name of the great men that are in the earth. Also I will ordain a place for my people Israel, and will plant them, and they shall dwell in their place, and shall be moved no more; neither shall the children of wickedness waste them any more, as at the beginning, And since the time that I commanded judges to be over my people Israel. Moreover I will subdue all thine enemies. Furthermore I tell thee that the Lord will build thee an house. 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. He shall build me a house, and I will establish his throne for ever. 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: But I will settle him in mine house and in my kingdom for ever: and his throne shall be established for evermore. (1 Chronicles 17:7-14, KJV)
Notice the language: "steadfast" "forever," "evermore." This is a profound promise that David's family is going to rule over Israel eternally as God's faithful people. Since Jesus counted himself as a descendant of King David, Christians take this to mean that God was finally following through with the "eternity" part of his promise through Jesus.