My brethren - David retains the modest phrase of a king not born in the purple, but raised from the ranks of the people (see 1 Samuel 30:23; 2 Samuel 19:12). No later Jewish monarch would have thus addressed his subjects.
The footstool of our God - David views the ark as God’s “footstool,” because he was enthroned above it visibly in the Shechinah, or luminous cloud, present from time to time above the mercy seat and between the cherubim (compare the marginal references).