so called without the negative connotations

Question

The context is a book on the interactions of pagans and Christians in antiquity. Sallustius is a pagan, but he put together a catechism of various pagan beliefs, which some of his contemporaries found extremely unHellenistic (a catechism normally being something associated with the Christians).

The following are the options for my sentence so far, but I am not satisfied. I figure most people hearing 'so-called' think of the negative connotations and I'm not crazy about the style of the last sentence, but will probably use it if all else fails:

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – a so-called “catechism”.

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – which was known as a "catechism".

As far as research goes I've checked thesaurus.com but didn't find anything suitable.

I'm looking for a word or a phrase that could fit in the gap instead of 'so-called' or 'which was known as'.

Sample sentence:
Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – a _________ “catechism”.

OR

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – ________ “catechism".

It's a translation, so there are certain restrictions on how much I can change the sentence The word-for-word translation would be: which one called a catechism.

Answer 1

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – a "catechism".

English can feel abrupt, and the current trend is for "strong" writing which often feels even more abrupt.

The side-effect is that longer sentences seem to suggest layers of subtlety or hidden meaning.

...which was known as a "catechism".

This phrase feels like a lead-in. It makes me expect the definition of "catechism" in the next sentence. If that is true, this phrase is perfect.

Answer 2

veritable adjective
: being in fact the thing named and not false, unreal, or imaginary —often used to stress the aptness of a metaphor
^{https://www.merriam-webster.com/dictionary/veritable}

It adds perhaps unwanted emphasis, but veritable is a commonly-used word in this context.

Answer 3

I suggest simply

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – which he called a “catechism".

or if you like

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – calling it a “catechism".

Which of course presupposes that Sallustius himself used that word. I did not find information if he did or not in your question. But if not, there is a remedy:

Sallustius even went a step further, compiling a handbook of its teachings – On the gods and the world – which people of his time called a “catechism".

Answer 4

If you want to be accurate and avoid any misunderstanding, you will probably want to take note of what "catechism" actually means to people who hear it. Some dictionaries, such as Oxford Living Dictionaries, only list definitions related to Christian use, and focuses on the question and answer form; Sallustius' work is not. The American Heritage Dictionary's primary definition relates to Christianity and emphasises the question-answer format:

1.
a. A text summarizing the basic principles of a Christian denomination, usually in question-and-answer form.
b. Formal indoctrination in the tenets of a Christian denomination; catechesis.
American Heritage Dictionary

Collins English Dictionary only gives definitions for a question-answer format of instruction:

n
1. (Ecclesiastical Terms) instruction by a series of questions and answers, esp a book containing such instruction on the religious doctrine of a Christian Church
2. rigorous and persistent questioning, as in a test or interview
Collins English Dictionary

The Wikipedia article on catechism strongly focuses on Christianity, and especially the question-answer form of instruction. For example under "Secular catechisms" are listed various works that are in the question and answer form, such as "Principles of Communism".

Principles of Communism is a brief 1847 work written by Friedrich Engels, the co-founder of Marxism. It is structured as a catechism, containing 25 questions about communism for which answers are provided.
Principles of Communism

I'm not going to go through every dictionary, but "catechism" also has a more generalised meaning, specifically generalised in two important factors:

1) non-Christian.
2) In a form not in a question-answer format of instruction.

Granted that "catechism" can have the general meaning of doctrinal instruction, and that it need not be particularly in question-answer form, then it means that there are different types of catechism, and therefore you may refer to Sallustius' work as:

"a type of catechism"

Interestingly, I found his work on Amazon, and also on Wikipedia, so I don't know what the original source is, but they both describe the work in the following terms:

Sallustius or Sallust was a 4th-century writer, a friend of the Roman Emperor Julian. He wrote the treatise On the Gods and the Cosmos, a kind of catechism of 4th-century Hellenic paganism. Sallustius

Also, I'm not sure you've seen the work yourself, but here it is. For the part I read, it seems quite interesting.
On Gods and the World

Also, just for the sake of answering your question in your title, an alternative for "so-called" can be "putative", meaning, "supposed" or "considered as".