I’ve looked at the word fast before, but I was specifically focusing on the adjective/adverb form (and why we don’t add -ly to it anymore). Today I wanted to take a look at the verb/noun form. Seems appropriate as we enter Holy Week, the end of the period of Lenton fasting, which contains one of the two days traditionally requiring a fast (Good Friday). 😁

As a quick reminder, the adj/adv form originally meant “firmly fixed.” This is preserved today in steadfast. A reminder I have to make, because the noun/verb meaning is from the same root and indeed carries much the same meaning.
From the Old English faesten, the word originally meant “to make firm; establish, confirm, pledge.” So let’s trace that a bit, shall we? “Make firm” easily moved into “to have firm control of oneself” and “confirming” or “pledging” similarly are necessary in order to abstain from something for religious reasons. So to fast was to hold oneself in observance of something…especially by abstaining from something…especially food. 
It definitely originated as a ritual tied to faith, but soon became the word used for any abstinence, whether it was for religious reasons or not. Hence, of course, breakfast being the first meal of the day, when we break the fast of the night.