Malachi 3:1

The first words of this chapter seem a direct answer to the profane atheistical demand of the scoffers of those days which closed the foregoing chapter: Where is the God of judgment? To which it is readily answered, "Here he is; he is just at the door; the long-expected Messiah is ready to appear; and he says, For judgment have I come into this world, for that judgment which you have so impudently bid defiance to." One of the rabbin says that the meaning of this is, That God will raise up a righteous King, to set things in order, even the king Messiah. And the beginning of the gospel of Christ is expressly said to be the accomplishment of this promise, with which the Old Testament concludes, Mar 1:1, Mar 1:2. So that by this the two Testaments are, as it were, tacked together, and made to answer one another. Now here we have, I. A prophecy of the appearing of his forerunner John the Baptist, which the prophet Isaiah had foretold (Isa 40:3), as the preparing of the way of the Lord, to which this seems to have a reference, for the words of the latter prophets confirmed those of the former: Behold, I will send my messenger, or I do send him, or I am sending him. "I am determined to send him; he will now shortly come, and will not come unsent, though to a careless generation he comes unsent for." Observe, 1.

— Matthew Henry's Concise Commentary (public domain)