Adam Clarke on Malachi 3:6

I am the Lord, I change not - The new dispensation of grace and goodness, which is now about to be introduced, is not the effect of any change in my counsels; it is, on the contrary, the fulfillment of my everlasting purposes; as is also the throwing aside of the Mosaic ritual, which was only intended to introduce the great and glorious Gospel of my Son.

And because of this ancient covenant, ye Jews are not totally consumed; but ye are now, and shall be still, preserved as a distinct people - monuments both of my justice and mercy.