The terms of this metaphor are not to be taken narrowly. The word "diner" corresponds to the evening meal in most cases but it refers also in a colloquial register to the main meal in the day, TLFi.

• C.− Fam. [Sans référence à une heure précise] Prendre le repas principal de la journée, manger.

Similarly, the word "souper" does mean "have the evening meal", but that is not much used nowadays, TLFi.

• A. − Vieilli ou région. (notamment Belgique, Canada, Est). Repas habituel du soir. Synon. usuel dîner.

"Souper" is also the verb for a meal that very few people have regularly nowadays and which takes place late in the evening, some hours after diner, and often after some type of entertainement TLFi.

• B. − Repas fin, collation pris(e) à une heure avancée de la nuit après le spectacle ou au cours d’une soirée.

From this state of affairs relative to the vocabulary we could take the first verb to mean "to have the the evening meal" and the second "to have the late evening meal" or we could take the first as "to have the main meal in the day" (midday meal) and the second "to have the evening meal". There is however some inconsistency in the use of the registers, but "le matin", which extends at most to 12 o’clock can never correspond to the time when people "dinent" without this use of the French verb "diner" being colloquial.

A consistent use of registers is given by the first option, but then there remains a little incoherence in the corresponding times.

Your understanding is right but your translation of:

Le matin catholique et le soir idolâtre.

is incorrect. The meaning is actually:

In the morning (being) Catholic, in the evening (being) miscreant.

These verses are an epigram about Simon-Joseph Pellegrin from the poet Rémy. It has been translated to:

Catholic in the morning and idolater in the evening,
he dined from the altar¹ and supped from the theatre.

This was written in the 18th century, at a time when the names of the meals hadn’t shifted their meanings in France but kept the one still used in most French speaking areas outside France. Even in some parts of France, saying dîner for lunch and souper for dinner was still done less than a century ago.

Etymologically, dîner and déjeuner are actually siblings both coming from the Latin disjejunare, literally "break fast".

This proverb from the 12th century clearly shows dîner started to be a morning meal :

Lever à cinq, dîner à neuf, souper à cinq, coucher à neuf, Font vivre d’ans nonante-neuf.

Wake up at five o’clock, breakfast at nine o’clock, dinner at five o’clock, will make you live ninety-nine years.

Note that instead of déjeuner/dîner, many restaurants use the non ambiguous and shorter midi/soir in their menu.

Note also that saying dîner is used colloquially here, as another answer states, is nonsensical.

¹_{It seems the original sentence was reading il dînait de l’autel, not de l’église}