The word is originally derived from the Greek adamas, meaning unbreakable or unflexible, and used as the name of a hypothetical hardest-possible substance, or in a metaphorical sense to refer to anything unalterable, such as Hades, god of death and king of the underworld.
It’s not hard to see how the word came to be used in its modern form in English. What’s curious though, is that using it in this abstract sense, to refer to beliefs, is actually very recent, dating to the early 20th century.
The word has been around for much longer than that though, entering Middle English around the 13th century via Old French. For those 800 years or so, it was used very much as adamas had been in Greek, often in relation to Greek myths, and at times in a metaphorical sense from which our modern use is clearly derived. John Dryden wrote, in the final book of his translation of Ovid’s Metamorphoses, for example:
There, on perennial design’d,
The various fortunes of your race you’ll find:
You can also find adamant or adamantine in works by authors as varied as Shakespeare, Edmund Spenser, Jonathan Swift, and J.R.R Tolkein.
Perhaps the most famous fictional use of a word directly derived from adamant is adamantium, a fictional, nigh-indestructible metal featuring in comics published by Marvel. It’s probably best-known as the metal that coats the skeleton of the X-Man Wolverine, though I’m reliably informed that it first appeared in 1966 as the substance from which the outer shell of Ultron, an evil robot, was made.
Which of course raises once more the eternal question: what happens when the adamantine sickel of Cronos meets the adamantium body of Ultron!?