Bailey's translation of section 123 of the Letter to Menoeceus is not to be relied upon, and I will always recommend reviewing Don's translation and commentary starting on page 29:
Quoteθεὸν is transliterated theon and is where English gets theology and atheist. τὸν θεὸν is singular, but, singular or plural, this can refer to a god, the gods, or the divine in general.
However, David Sedley in his paper "Epicurus' Theological Innatism" places significance on the singular construction. Sedley proposed each person creates their own "god" which is why he stressed the importance of the singular form. Sedley's paper is recommended reading. So, where the word is singular, I will try to translate it as such as to not obscure the semantics.
So I would amend Bailey's translation for clarity in the following way;
QuoteFirst of all believe that [a] god is a being immortal and blessed, even as the common idea of a god is engraved on men’s minds, and do not assign to
him[such a being] anything alien tohis[its] immortality or ill-suited tohis[its] blessedness: but believe abouthim[such a being] everything that can upholdhis[its] blessedness and immortality.
Now, why does Epicurus use the singular here, and the plural subsequently? As Don points out, Dr. Sedley has a paper speaking to this question that I need to review. For now, I think we can look to the Letter to Herodotus for an answer:
QuoteFirst of all, Herodotus, we must grasp the ideas attached to words, in order that we may be able to refer to them and so to judge the inferences of opinion or problems of investigation or reflection, so that we may not either leave everything uncertain and go on explaining to infinity or use words devoid of meaning.
[38] For this purpose it is essential that the first mental image associated with each word should be regarded, and that there should be no need of explanation, if we are really to have a standard to which to refer a problem of investigation or reflection or a mental inference. And besides we must keep all our investigations in accord with our sensations, and in particular with the immediate apprehensions whether of the mind or of any one of the instruments of judgment, and likewise in accord with the feelings existing in us, in order that we may have indications whereby we may judge both the problem of sense perception and the unseen.
The"first mental image" of a god is of 'a being blessed and incorruptible', and that image is a type, according to one definition of that word:
Quote2. a person or thing symbolizing or exemplifying the ideal or defining characteristics of something.
Now that we have this image in our minds, we can hold other claims, descriptors, delimiters, and representations up to it, to test whether they hold good or not. A jealous, petty, angry, vindictive god does not accord with the mental image of blessedness, and so we can disregard such a view of the gods.
Note, though, that the 'first mental image' of a thing is NOT an ideal platonic form of the class of things it represents, and it has no being outside of our minds. There is no perfect and unchanging realm of pure being. And our souls do not innately possess knowledge of these mental images to be 'recollected' through the study of logic and geometry; these mental images are formed as impressions by repeated exposure. They are a distillation of experience, and not something external and eternal that precedes experience.
If I've made a mistake here, I trust that Don and Bryan will correct me!