Keeper of Secrets

Probably because some people have an unwarranted opposition to “Archprelate-Goddess”.

Wikipedia has an alternate explanation:

en.wikipedia.org/wiki/Secretary_(title)

Secretary is a title often used in organizations to indicate a person having a certain amount of authority, power, or importance in the organization. Secretaries announce important events and communicate to the organization. The term is derived from the Latin word secernere, “to distinguish” or “to set apart”, the passive participle (secretum) meaning “having been set apart”, with the eventual connotation of something private or confidential, as with the English word secret. A secretarius was a person, therefore, overseeing business confidentially, usually for a powerful individual (a king, pope, etc.).

The official title of the leader of most communist and socialist political parties is the “General Secretary of the Central Committee” or “First Secretary of the Central Committee”. When a communist party is in power, the general secretary is usually the country’s de facto leader (though sometimes this leader also holds state-level positions to monopolize power, such as a presidency or premiership in order to constitute de jure leadership of the state), such as China, North Korea, Vietnam, Laos and Cuba.[1]

In England, the term secretarius was used “from the beginning of the thirteenth century in the varying meanings of a confidential clerk, an ambassador, or a member of the king’s council”.[2] In the fourteenth century, the title became strongly associated with the keeper of the king’s signet.[2] From the Renaissance to the late 19th century, men involved in the daily correspondence and the activities of the powerful assumed the title of secretary. With time, like many titles, the term was applied to more and varied functions, leading to compound titles to specify the authority associated with its use, like general secretary or financial secretary.

In some countries, such as the United States, the term secretary is used to indicate the holder of a cabinet-level post. There are a number of popular variations of the title used to indicate that the secretary in question has a high degree of authority, such as general secretary (or, following usage in the Norman language, secretary-general), first secretary, and executive secretary. A woman holding such an office is often addressed informally as “Madam Secretary”.

That gets one to the next-to-most powerful spot, but it’s not clear from that how the term made it to the most powerful. I’m reading between the lines a bit here, but I suspect that the idea is that it might have political value to present oneself as being a functionary rather than a ruler. That is, if you decide to style yourself “supreme dictator for life”, even if thst’s what you are, it might not sell as well with the public.

en.wikipedia.org/…/General_Secretary_of_the_Commu…

The General Secretary of the Central Committee of the Communist Party of the Soviet Union[a] was the leader of the Communist Party of the Soviet Union (CPSU). From 1924 until the country’s dissolution in 1991, the officeholder was the recognized leader of the Soviet Union.[2][3] Prior to Joseph Stalin’s accession, the position was not viewed as an important role in Vladimir Lenin’s government[4][5] and previous occupants had been responsible for technical rather than political decisions.[6]

Officially, the General Secretary solely controlled the Communist Party directly.[dubious – discuss] However, since the party had a monopoly on political power, the General Secretary de facto had executive control of the Soviet government. Because of the office’s ability to direct both the foreign and domestic policies of the state and preeminence over the Soviet Communist Party, it was the de facto highest office of the Soviet Union.

EDIT: I’d note that many countries with a parliamentary system are led by a “prime minister”, though that term really derived from when the minister had a comparable role, acting on behalf of the monarch, and as power moved away from the monarchy, effectively supplanting them as leader. So it’s not so unique for a “secretary” to do the same, I suppose.

www.etymonline.com/word/prime minister

minister

The political sense of “high officer of the state