He shall abide before God for ever - That is, perpetually; without danger of change, or of being driven into exile. This may allude, however, to the hope which David had that he would always live with God in a higher world - a world where there would be no danger of change or banishment. His restoration to his home, to his throne, and to the privileges of the sanctuary, he may have regarded as an emblem of his ultimate reception into a peaceful heaven, and his mind may have glanced rapidly from the one to the other. On earth, after his restoration, he would have no fear that he would be banished again; in heaven, of which such a restoration might be regarded as an emblem, there could be no change, no exile.
O prepare mercy and truth - literally, divide, or divide out; then, allot or appoint; and then, make ready or prepare. The prayer is, that God would measure out to him, or impart to him, such favor that this desire of his heart would be realized. On the phrase mercy and truth, see Psalms 25:10, note; Psalms 57:3, note; Psalms 57:10, note.
Which may preserve him - They will preserve him. That is, the manifestation of such mercy and truth would make his permanent occupancy of his throne on earth, and his ultimate reception into heaven, secure.