Agatharchides of Cnidus is one of many major Greek and Roman authors who we only know via textual fragments preserved in the works of others that have come down to us. In the case of Agatharchides fragments of his only known work 'On the Erythraean Sea' (from the second century BC) have come down to us via Strabo, Diodorus and Photius. (1) The latter was a prolific Byzantine author and compiler who was responsible for saving a considerable amount of ancient material. That Agatharchides was quoted and cited extensively by two near contemporary authorities Strabo and Diodorus who are well regarded by scholars today indicates how well-known and well-received Agatharchides' work was in the ancient Greek world.
In one of his discussions on the subject of (oriental) superstition Agatharchides tells us that the city of Jerusalem fell to Ptolemy I (sometimes known as Ptolemy Soter) of Egypt due to the jews believing they were forbidden to fight on the Sabbath. (2)
Agatharchides tells us thus:
'So because, instead of guarding their city, these men observed their senseless rule, the city received a harsh master, and their law was shown to be a foolish custom.' (3)
The point of Agatharchides is to the point out that jewish religious beliefs are inherently idiotic, because they are fundamentally flawed in that their observance requires their devotees to undertake to do no work on a specific day each week (Shabbos) even when they are at war.
Leading to the enemy predictably taking advantage of his happy situation and promptly moving to take their cities of the jews and slaughter their armies without them lifting a hand against them (for fear of the wrath of Yahweh descending upon them [apparently they were more scared of Yahweh than they were of Greek and Egyptian soldiers razing their city to the ground and massacring them]).
Essentially Agatharchides regards the jews as the people of crass superstition par extraordinaire - which is why he relates the story of the fall of Jerusalem to Ptolemy I - and a people who should be looked to not as an example of all that is good in religion, but rather all that is bad. Since any religion that requires its worshipers to be willing butchered if an enemy is so impertinent as to attack them on the wrong day must be regarded; and rightly so, as inherently irrational and thus superstitious (because it isn't founded upon reason or logic).
Thus we can see that Agatharchides is referring to the jews as the pre-eminent of all people in their superstitious beliefs as well as being supreme in the sheer irrationality of those superstitions.
References
(1) For the only English compilation and translation of the text see Stanley Burstein (Ed. and Trans.), 1989, 'Agatharchides of Cnidus: On the Erythraean Sea', 1st Edition, The Hakluyt Society: London
(2) For a summary of the context for this comment see Erich Gruen, 2002, 'Diaspora: Jews amidst Greeks and Romans', 1st Edition, Harvard University Press: Cambridge, p. 49
(3) Translation from Max Radin, 1915, 'The Jews among Greeks and Romans', 1st Edition, Jewish Publication Society of America: Philadelphia, p. 178