Verily I say unto you,.... An usual form of introducing something of moment to raise attention to it, and to ascertain the truth of it:

all these things shall come upon this generation; all the things which Christ had foretold should come to pass in the present age; as that the apostles and ministers of the word he should send to them, some of them they would kill and crucify, and others they would scourge in their synagogues, or persecute from place to place; and all the horrible murders and bloodshed in any age, committed by that people, would be placed to the account of the men of that generation; and the guilt of them imputed to them, and the punishment due unto them be inflicted on them. And which came to pass, and had its full accomplishment about forty years after this, in the utter destruction of Jerusalem, and the whole nation; so that many now living were personally involved in that temporal ruin, as well as escaped not the damnation of hell, Mat 23:33.

Wherefore, behold, I send unto you prophets, and wise men, and scribes--The I here is emphatic: "I am sending," that is, "am about to send." In Luk 11:49 the variation is remarkable: "Therefore also, said the wisdom of God, I will send them," &c. What precisely is meant by "the wisdom of God" here, is somewhat difficult to determine. To us it appears to be simply an announcement of a purpose of the Divine Wisdom, in the high style of ancient prophecy, to send a last set of messengers whom the people would reject, and rejecting, would fill up the cup of their iniquity. But, whereas in Luke it is "I, the Wisdom of God, will send them," in Matthew it is "I, Jesus, am sending them"; language only befitting the one sender of all the prophets, the Lord God of Israel now in the flesh. They are evidently evangelical messengers, but called by the familiar Jewish names of "prophets, wise men, and scribes," whose counterparts were the inspired and gifted servants of the Lord Jesus; for in Luke (Luk 11:49) it is "prophets and apostles."

unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar--As there is no record of any fresh murder answering to this description, probably the allusion is not to any recent murder, but to Ch2 24:20-22, as the last recorded and most suitable case for illustration. And as Zacharias' last words were, "The Lord require it," so they are here warned that of that generation it should be required.