We may here see,
I. How apt many of Christ's own disciples are to idolize things that look great, and have been long looked upon as sacred. They had heard Christ complain of those who had made the temple a den of thieves; and yet, when he quitted it, for the wickedness that remained in it, they court him to be as much in love as they were with the stately structure and adorning of it. One of them said to him, "Look, Master, what manner of stones, and what buildings are here, Mar 13:1. We never saw the like in Galilee; O do not leave this fine place."
II. How little Christ values external pomp, where there is not real purity; "Seest thou these great buildings" (saith Christ), "and admirest thou them? I tell thee, the time is at hand when there shall not be left one stone upon another, that shall not be thrown down," Mar 13:2. And the sumptuousness of the fabric shall be no security to it, no nor move any compassion in the Lord Jesus towards it. He looks with pity upon the ruin of precious souls, and weeps over them, for on them he has put great value; but we do not find him look with any pity upon the ruin of a magnificent house, when he is driven out of it by sin, for that is of small value with him. With what little concern doth he say, Not one stone shall be left on another! Much of the strength of the temple lay in the largeness of the stones, and if these be thrown down, no footstep, no remembrance, of it will remain. While any part remained standing, there might be some hopes of the repair of it; but what hope is there, when not one stone is left upon another?
III. How natural it is to us to desire to know things to come, and the times of them; more inquisitive we are apt to be about that than about our duty. His disciples knew not how to digest this doctrine of the ruin of the temple, which they thought must be their Master's royal palace, and in which they expected their preferment, and to have the posts of honour; and therefore they were in pain till they got him alone, and got more out of him concerning this matter. As he was returning to Bethany therefore, he sat upon the mount of Olives, over against the temple, where he had a full view of it; and there four of them agreed to ask him privately, what he meant by the destroying of the temple, which they understood no more than they did the predictions of his own death, so inconsistent was it with their scheme. Probably, though these four proposed the question, yet Christ's discourse, in answer to it, was in the hearing of the rest of the disciples, yet privately, that is, apart from the multitude. Their enquiry is, When shall these things be? They will not question, at least not seem to question, whether they shall be or no (for their Master has said that they shall), but are willing to hope it is a great way off. Yet they ask not precisely the day and year (therein they were modest), but say, "Tell us what shall be the sign, when all these things shall be fulfilled? What presages shall there be of them, and how may we prognosticate their approach?"

See what manner of stones - Josephus says, Ant. b. xv. chap. 11: "That these stones were white and strong, Fifty feet long, Twenty-Four broad, and Sixteen in thickness." If this account can be relied on, well might the disciples be struck with wonder at such a superb edifice, and formed by such immense stones! The principal contents of this chapter are largely explained in the notes on Matt. 24, and to these the reader is requested to refer.

And as he sat upon the Mount of Olives,.... On the east of Jerusalem:

over against the temple: where he could have a full view of it; the eastern wall of the temple being lower than the rest; See Gill on Mat 24:3.

Peter, and John, and James, and Andrew, asked him privately; apart from the rest of the disciples, they being, especially the first three, his favourites, and very familiar with him.