It seems strange that a Philistine like Achish would swear by the LORD. In doing so he makes use of a well known oath formula (cf. 1 Samuel 14:39).1 Most likely he does so as a matter of courtesy and respect. In a polytheistic culture, people believed in the existence of many gods and were happy to acknowledge other gods
as and when it suited them.2 It is possible however that he developed an interest in the LORD based on what he saw of David and the success that he enjoyed.3
6 Akisch appela David, et lui dit: L'Eternel est vivant! tu es un homme droit, et j'aime à te voir aller et venir avec moi dans le camp, car je n'ai rien trouvé de mauvais en toi depuis ton arrivée auprès de moi jusqu'à ce jour; mais tu ne plais pas aux princes.