Theophylact of Ohrid

Since the Lord spoke much about the desolation of Jerusalem, for example: "your house is left unto you desolate" (Matt. 23:38), the disciples marveled that such magnificent and beautiful buildings were to perish, and therefore they showed Him the striking splendor of the temple. But He foretold that, nevertheless, of all this not even one stone would be left upon another. Against this, some say that many remnants of the ancient city of Jerusalem have survived, and thus they attempt to make Christ out to be a liar. …

Commentary on Mark ↗

Augustine of Hippo

In answer to the disciples, the Lord tells them of things which were from that time forth to have their course; whether He meant the destruction of Jerusalem which occasioned their question, or His own coming through the Church, (in which He ever comes even unto the end, for we know that He comes in His own, when His members are born day by day,) or the end itself, in which He will appear to judge the quick and the dead.

Catena Aurea by Aquinas ↗

Bede

Because the Lord, when some were praising the buildings of the temple, had plainly answered that all these were to be destroyed, the disciples privately enquired about the time and the signs of the destruction which was foretold; wherefore it is said: And as he sat upon the mount of Olives, over against the temple, Peter and James and John and Andrew asked him privately, Tell us when shall these things be? and what shall be the sign when all these things shall be fulfilled. …

Catena Aurea by Aquinas ↗