Ye be witnesses unto yourselves - The emphasis, here, lies in the words "to yourselves." It is an appeal to their conscience. It was not by their building the tombs that they were witnesses that they were the children of those who killed the prophets; but that, in spite of all this pretence of piety, under all this cloak of profession, they knew in their consciences, and were witnesses to themselves, that it was mere hypocrisy, and that they really approved the conduct of those who slew the prophets.
Children of them - Resembling them; approving their conduct; inheriting their feelings. See the notes at Matthew 1:1. They not only showed that they were descended from them, but that they possessed their spirit, and that, in similar circumstances, they would have done as they did.

Ye be witnesses - Ye acknowledge that ye are the children of those murderers, and ye are about to give full proof that ye are not degenerated.
There are many who think that, had they lived in the time of our Lord, they would not have acted towards him as the Jews did. But we can scarcely believe that they who reject his Gospel, trample under foot his precepts, do despite to the Spirit of his grace, love sin, and hate his followers, would have acted otherwise to him than the murdering Jews, had they lived in the same times.

Wherefore ye be witnesses unto yourselves,.... Or "against yourselves", as the Syriac reads; for what they said was a plain acknowledgment, and a full confession, what their fathers had done, and whose offspring they were; and from whom better things were not to be expected; since they were their fathers' own children, and of the same temper and disposition with them:
that ye are the children of them that killed the prophets. They plainly owned, that their fathers killed the prophets, and that they descended from them; though they meant not so much to reproach, their ancestors, as to give themselves a greater character; yet it did not with those, that knew them; not with our Lord: for as their own words testified against them, that they were a seed of evildoers; their practices showed them to be of the same spirit and principles with their progenitors.

Wherefore ye be witnesses . . . children of them which killed the prophets. They demonstrated by their hostility to Christ, by their plots and false charges, and would soon show by their murder of the Lord, that they had just the same spirit as their fathers who slew Isaiah, persecuted Jeremiah, and shed the blood of Zacharias between the altar and the temple. They were therefore their spiritual children as well as their descendants. It adds to the vividness of this denunciation that from the temple area where they were standing the crest of Olivet rose distinctly at the distance of half a mile, and upon it were clearly visible the white sepulchers of the prophets which they had rebuilded, among them the tomb of Zacharias, who is named just below as slain between "the temple and the altar."

Wherefore ye testify against yourselves - By your smooth words as well as devilish actions: that ye are the genuine sons of them who killed the prophets of their own times, while they professed the utmost veneration for those of past ages.
From Matthew. 23:3-30 is exposed every thing that commonly passes in the world for religion, whereby the pretenders to it keep both themselves and others from entering into the kingdom of God; from attaining, or even seeking after those tempers, in which alone true Christianity consists. As, Punctuality in attending on public and private prayer, Matthew 23:4-14. Zeal to make proselytes to our opinion or communion, though they have less of the spirit of religion than before, Matthew 23:15. A superstitious reverence for consecrated places or things, without any for Him to whom they are consecrated, Matthew 23:16-22. A scrupulous exactness in little observances, though with the neglect of justice, mercy, and faith, Matthew 23:23-24. A nice cautiousness to cleanse the outward behaviour, but without any regard to inward purity, Matthew 23:25-26. A specious face of virtue and piety, covering the deepest hypocrisy and villany, Matthew 23:27-28 A professed veneration for all good men, except those among whom they live.