What does 1 Kings 12:19 mean?

"To this day," as noted here, refers to when this text was originally written. The single book we now call 1 and 2 Kings is a sequel to the single book we now call 1 and 2 Samuel. Kings tells the story of Solomon's reign over the nation of Israel, the split of the nation into the northern and southern kingdoms, and the lifespans of those kingdoms with an emphasis on the history of the northern kingdom of Israel. The book was likely compiled and written between 561 and 538 BC, during the Babylonian captivity and before the return to Jerusalem. At least 160 years before, the Assyrians had destroyed Israel. And still, nearly 300 years after the split of the nation, the author still frames the story as Israel's rebellion against the house of David.

God intended the house of David—the line of kings directly descended from David—to be continuous and last forever (2 Samuel 7:16). The forever part is God's unconditional promise to David. The continuous part is dependent on the individual kings' continued obedience to God. God had told Solomon, "You shall not lack a man on the throne of Israel" (1 Kings 9:5) unless he or his sons break God's commandments and serve other gods. Not only would David's dynasty be threatened, but the nation would crumble into ruins (1 Kings 9:6–9).

Of course, Solomon did break God's commandments. He did worship other gods (1 Kings 11:1–8). So God told him that his adversary would take part of the kingdom from his son (1 Kings 11:9–13).

Several things are true at the same time. Solomon broke the conditional covenant God made with David, nullifying God's offer of a contiguous dynasty. God chose to take most of the kingdom from Solomon's son. Rehoboam promised the northern tribes increased cruelty (1 Kings 12:12–15). The northern tribes are in rebellion.

Solomon, Rehoboam, and the northern tribes are in sin. The nation is split, and Jeroboam will be king over the northern tribes. This is God's judgment.