William Burkitt on Luke 21:5

Our blessed Saviour being now ready to depart from the temple, nevermore after this entering into it, and his disciples showing him, with wonder and admiration, the magnificent structures and buildings thereof, apprehending that in regard of its invincible strength it could not be destroyed; not considering, that sin will undermine and blow up the most magnificent and famous structures; for sin brings cities and kingdoms, as well as particular persons, to their end. Not one stone, says Christ, shall be left upon another; which threatening was exactly fulfilled after Christ's death, when Titus the Roman emperor destroyed the city, burnt the temple and Turnus Rufus, the general of his army, ploughed up the very foundation on which the temple stood; thus was the threatening of God fulfilled, Zion shall be ploughed as a field, and Jerusalem shall become an heap. Jer 26:18

Learn hence,

1. That sin has laid the foundation of ruin in the most flourishing cities and kingdoms; Jerusalem, the glory of the world, is here by sin threatened to be made a desolation.

2. That the threatenings of God are to be feared, and shall be fulfilled, whatever appearing improbabilities there may be to the contrary. 'Tis neither the temple's strength nor beauty that can oppose or withstand God's power.