Hypsistarians, i.e. worshippers of the Hypsistos (Greek: Ὕψιστος, the "Most High" God), is a term that appears in documents that date from around 200 BC to around AD 400, referring to various groups mainly in Asia Minor (Cappadocia, Bithynia and Pontus) and the Black Sea coasts that are today part of Russia.
The names Hypsianistai, Hypsianoi first occur in Gregory of Nazianzus (Orat., xviii, 5) and the name Hypsistianoi in Gregory of Nyssa (Contra Eunom., II), i. e. about AD 374, but a great number of votive tablets, inscriptions and oracles of Didyma and Klaros establish beyond doubt that the cult of the Hypsistos (Hypsistos, with the addition of Theos 'god' or Zeus or Attis, but frequently without addition) as the supreme God was widespread in the countries adjacent to the Bosphorus (cf. Acts 16:17, "these men are servants of the most high God" — oracle of the pythia at Philippi).
Contemporary Hellenistic use of ὕψιστος (hýpsistos) as a religious term appears to be derived from and compatible with the term as it had much earlier appeared in the Septuagint. (Greek ὕψιστος translating Hebrew elyon עליון English "highest".)
In the Septuagint the root word "hypsisto-" occurs more than fifty times as a substitution for the Tetragrammaton (the name of God) or in direct relation to Him (most often in the Psalms, Daniel, and Sirach).
It is possible that the native Cappadocian cult of Zeus Sabazios slowly integrated into the cult of Jahve Sabaoth practiced by the numerous and intellectually predominant Jewish colonies, and that associations (sodalicia, thiasoi) of strict monotheists formed, who fraternized with the Jews, but who considered themselves free from the Mosaic Law. The importance and exalted ideas of these associations can be gathered from the fact that when someone asked Apollo of Klaros whether the Hypsistos alone was without beginning and end, he answered: "He is the Lord of all, self-originated, self-produced, ruling all things in some ineffable way, encompassing the heavens, spreading out the earth, riding on the waves of the sea; mixing fire with water, soil with air and earth with fire; of winter, summer, autumn and spring, causing the changes in their season, leading all things towards the light and settling their fate in harmonious order."
The existence of Hypsistarians may have contributed to the astounding swiftness of the spread of Christianity in Asia Minor; yet not all of them accepted the new faith, and small communities of monotheists, neither Christians nor Jews, continued to exist, especially in Cappadocia. The father of Gregory of Nazianzus belonged to such a sect in his youth, and they are described in his panegyric written by his son. Such Hypsistarians rejected idols and non-Abrahamic sacrifices, and acknowledged the Creator (pantokrator) and the Most High, to whom however, in opposition to the Christians, they refused the title of "Father"; they had some customs in common with the Jews (the keeping of the Sabbath, the distinctions of food) but they rejected circumcision.
Persius (34-62) may have had Hypsistarians in view when he ridiculed such hybrid religionists in Satire v, 179–84, and Tertullian (c. 160 – c. 225 AD) seems to refer to them in Ad nationes, I, xiii. The claim that Hypsistarians continued to exist until the ninth century relies on a mistaken interpretation of Nicephorus Const., "Antirhet. adv. Const. Copr.", I, in Migne, PG, col. 209. Hypsistarians are probably referred to under the name Coelicoloe in a decree of the Emperors Honorius and Theodosius II (AD 408), in which their places of worship are transferred to the Christians.
After describing his difficulties with mainstream religion, Goethe laments that
...I have found no confession of faith to which I could ally myself without reservation. Now in my old age, however, I have learned of a sect, the Hypsistarians, who, hemmed in between heathens, Jews and Christians, declared that they would treasure, admire, and honour the best, the most perfect that might come to their knowledge, and inasmuch as it must have a close connection to the Godhead, pay it reverence. A joyous light thus beamed at me suddenly out of a dark age, for I had the feeling that all my life I had been aspiring to qualify as a Hypsistarian. That, however, is no small task, for how does one, in the limitations of one's individuality, come to know what is most excellent?
Their ideas about God derived from a syncretized monotheism, combining elements of the Cappadocian cult of Zeus Sabazios with the Jewish God Yahweh Sabaoth. Hypsistarians accordingly amalgamated religious practices from paganism and Judaism.
They rejected idols and pagan sacrifices, and acknowledged the Creator and Most High, to whom however, in opposition to the Christians, they refused the title of 'Father...'