The idea that the various nations worshiped basically the same deities but under different names and in different forms eventually led to the belief in a Supreme Being (the Greek expression is Hypsistos, the Highest One) comprising in its essence not only all the myriads of known and unknown deities, but above all those three or four Gods who, in the context of different religions, play the role of the Highest God (usually Zeus, Sarapis, Helios, and Iao=YHWH). This superdeity is addressed by appellations such as Hypsistos (Supreme) and the widespread "One-God" predication Heis Theos. Oracles typically proclaim particular Gods to be one and the same together with other Gods: "One Zeus, one Hades, one Helios, one Dionysus, One God in all Gods." (Pseudo-Justin, "Exhortation against the Greeks" 15=Orphic frag 239 [Macrobius, "Saturnalia" 1.18.17 quotes the first verse]) In one of the oracles, Iao, the God of the Jews, is proclaimed to be the God of Time (Olam-Aion), appearing as Hades in winter, Zeus in springtime, Helios in summer, and "Habros Iao" in autumn. (Macrobius, "Saturnalia" 1.18.20; see Peterson 1926: 243-4; Hengel 1969: 476-7; and the inscription Heîs Zeùs Sérapis Iaó [CIL 2 suppl 5665=Dunand 1975: 170]). These oracles and predications manifest a quest for the sole and supreme divine principle beyond the innumerable multitude of specific deities. This is typical of the "ecumenical age" and seems to correspond to efforts toward political unification (see Peterson 1935, 1951; Schindler 1978; Momigliano 1987; Dunand 1975; and Fowden 1993). The belief in the Supreme Being (Hypsistos) has a distinctly universalist character: "The sons of Ogyges call Me Bacchus, Egyptians think Me Osiris, Mysians name Me Phanaces, Indians regard Me as Dionysus, Roman rites make Me Liber, The Arab race thinks Me Adoneus, Lucaniacus the Universal God." (Ausonius "Epigrammata" #48 [trans White 1985]) This tradition of invoking the Highest God by the names given Him by the various nations expresses a general conviction in late antiquity about the universality of religious truth and the relativity of religious institutions and denominations and the conventionality of divine names. According to Servius, the Stoics taught that there is only one God, whose names merely differ according to actions and offices. Varro (1116-27 BCE), who knew about the Jews from Poseidonius, was unwilling to make any differentiation between Jove and YHWH because he was of the opinion that it mattered little by which name he was called as long as only the same thing was meant ("nihil interesse censens quo nomine nuncupetur, dum eadem res intelligatur"; "Antiquitates rerum divinarum", frag 16 Cardauns). Porphyry held the opinion that the names of the Gods were purely conventional. Celsus argued that "it makes no difference whether one calls God 'Supreme' [Hypsistos] or Zeus or Adonai or Sabaoth or Ammon such as the Egyptians do or Papaios as the Scythians." The name does not matter when it is evident what or who is meant.
“Monotheism and Polytheism” by Jan Assmann in Ancient Religions edited by Sarah Iles Johnston (p 27-8)