Please explain "until the times of the Gentiles be fulfilled" from Luke 21:24.
"But when you see Jerusalem surrounded by armies, then recognize that her desolation is near. Then those who are in Judea must flee to the mountains, and those who are in the midst of the city must leave, and those who are in the country must not enter the city; because these are days of vengeance, so that all things which are written will be fulfilled. Woe to those who are pregnant and to those who are nursing babies in those days; for there will be great distress upon the land and wrath to this people; and they will fall by the edge of the sword, and will be led captive into all the nations; and Jerusalem will be trampled under foot by the Gentiles until the times of the Gentiles are fulfilled" (Luke 21:24).
To understand a phrase, we must make sure we understand the context in which the phrase appears. Jesus is discussing the destruction of Jerusalem and giving his followers instructions so that they will not be caught in its destruction. The fall of Jerusalem was devastating. Josephus said the destruction of the city and temple was so complete that those who visited it afterward could hardly believe that it had ever been inhabited [Josephus, The Wars of the Jews, 7.1]. Those who did not die in the actual destruction of Jerusalem were hauled off into slavery and scattered through the Roman Empire.
Jesus declares that Jerusalem would be turned over to the Gentiles to be trampled by them until their time is completed. How much time that would be was left open. Some connect this to Romans 11:25, but the topic in Romans is completely different from what it is here in Luke 21. Rather Jesus is stating that just as God would use the Gentiles (the Romans) to bring judgment on Jerusalem, so too would the Gentiles face judgment when their time is complete.
This is not unusual. God has done this before. For example, Babylon was brought in to punish Judah (Habakkuk 1:6), and then it to was destroyed -- "they will be held guilty" (Habakkuk 1:11).