Heidegger's Bible Handbook: Malachi: Seal of the Prophets

4.  He is the seal of the Prophets.

Not incorrectly do the Hebrews call Malachi the חותם הנביאים, seal of the Prophets.  For, he is the last Prophet and θεόπνευστος/inspired writer.  He sealed the Old Testament, and place the finishing touch upon it.  He especially urges that final mandate to hold in memory the Mosaic Law and the promise of forerunner of the Christ, as the next and soon coming Prophet, and agreeing in his proclamation with the proclamation of Malachi.  But, even if in the following ages the Church of God also had its distinguished men and illustrious heroes, not one of them was furnished with the Prophetic call and authority, which is shown at greater length in its own place.