Jamieson, Fausset and Brown on Psalms 18:49-50

49-50. Paul (Ro 15:9) quotes from this doxology to show that under the Old Testament economy, others than the Jews were regarded as subjects of that spiritual government of which David was head, and in which character his deliverances and victories were typical of the more illustrious triumphs of David's greater Son. The language of Psa 18:50 justifies this view in its distinct allusion to the great promise (compare 2Sa 7:12). In all David's successes he saw the pledges of a fulfilment of that promise, and he mourned in all his adversities, not only in view of his personal suffering, but because he saw in them evidences of danger to the great interests which were committed to his keeping. It is in these aspects of his character that we are led properly to appreciate the importance attached to his sorrows and sufferings, his joys and successes.