God-fearers, Hypsistarians and Sabians
Fearers
In the Hebrew Bible, there is recognition of monotheistic non-Jewish worship of YHWH. Such worshippers fall into the category of yir'ei Hashem or "fearers of the Name." ("The Name" or HaShem is a Jewish euphemism for YHWH.)
Gentiles sympathetic to Second Temple Judaism were known as God-fearers. They are mentioned in the Christian New Testament and other contemporary sources such as synagogue inscriptions in diaspora Hellenistic Judaism.
The term God-fearers is therefore used to describe gentiles who attached themselves in varying degrees to Judaism without becoming total converts. They are referred to in the Christian New Testament's Book of Acts. Cornelius, for example, is described as “devout and God-fearing” (Acts 10: 1-2).
In Satires 14:96-106, the Roman poet Juvenal made a number of insulting references to God-fearers who “respect” the Sabbath:
“They worship nothing but the clouds and the sky. They think there is no difference between pork […] and human flesh. […] And with their habit of despising the laws of Rome, they study, observe, and revere the Judaic code, as handed down by Moses.”
Josephus and Tertullian also make reference to God-fearers.
Hypsistarians
Hypsistarians (i.e. worshippers of the Hypsistos or "Most High" God) is a term appearing in documents dated about 200 BC to about AD 400, referring to various groups mostly in Asia Minor. In the Greek translation of the Jewish scriptures, the word "hypsisto" is used more than fifty times as a title for YHWH. The existence of these Hypsistarians may help explain the swiftness of the spread of Christianity in Asia Minor. But not all of them accepted Christianity and small communities of monotheists, neither Christians nor Jews, continued to exist, especially in Cappadocia. They rejected idols, observed the Sabbath and kept kosher laws, but rejected circumcision.
Sabians
Not to be confused with Sabians or Sabaeans, another name for the Mandaeans.)
Sebomenoi or Sebeoi is a monotheistic community of God-fearers referred to in Greek manuscripts from the Hellenistic-Roman period. Modern research indicates that the Greek word TheoSeBeia is derived from the Hebrew ger ToShaB, which evolved into the word TSaBian or Sabian.
The Qur'an mentions briefly the Sabians as “people of the book,” like Jews and Christians. According to some Muslim authorities, Sabians used the book of Zabur in their religious observances. Most scholars believe Zabur to be the biblical book of Psalms.
Interestingly, Khalil Ibn Ahmad, who lived in the eighth century, said the Sabians "believe they belong to the prophet Noah [and] they read Psalms."
Al-Biruni, writing at the start of the eleventh century CE, said that the Sabians were "the remnants of the Jewish tribes who remained in Babylonia when the other tribes left it for Jerusalem in the days of Cyrus and Artaxerxe.”