Hear ye, and testify in the house of Jacob, saith the Lord GOD, the God of hosts,
Hear ye, and testify in the house of Jacob - i:e., unto the house of Jacob - i:e., against the house of Jacob. God calls on the same persons as in Amos 3:9 - namely, the pagan Philistines and the Egyptians-to witness with their own eyes Samaria's corruptions above described, so as that none may be able to deny the justice of Samaria's punishment (Maurer).
The God of hosts - having therefore all the powers of heaven and earth at command, and so being One calculated to strike terror into the hearts of the guilty, whom He threatens.