There is also the question of Fate in the singular. The Fates were typically portrayed as three women, the Moriai spoken of collectively, but not in the singular. Thus the concept of a single entity of "Fate" didn't exist. Nor would Classical Greek necessarily consider the Moriai to be responsible for the unavoidability of said "it" as, while they were the keepers of mans' thread (life span), it was Zeus who ultimately determined a man's fate. The Moriai were simply the weavers of the thread. Of course, there is the problem of differing interpretations. Not all Greek authors treat the Fates the same, sometimes giving them powers over man, other times just keeping the keepers of the threads.
The concept that something must be fate comes to us from a Christian concept of God's intervention in our lives - and thus broaches the whole idea of freewill verses divine plan. If our lives are part of a divine plan, how can we have freewill, the power to control our own lives? Philosophers have been debating this question for centuries (long before Christ, as Plato and Aristotle weigh in on the question).
Add to this our concepts of who the Fates were is only illuminated by the surviving literature. In Classical Greek literature, what we have are not the common tales, stories that everyone would know, but rather the works of literature which refer to these common tales obtusely, rather than in direct terms. Today, when we reference the "glass slipper" or "turning into a pumpkin", these refer to tales from our childhood, reference we just expect our audience to know and understand; they don't need explaining. However, to someone who doesn't grow up with the story of Cinderella, the concept of what I mean when I say "I turn into a pumpkin at midnight" is completely foreign to them. To the Classical Greeks, the Moriai must have been so common, so much a part of their culture, there was no reason to explain who and what they did. In this instance, they were very much a part of the fabric of Classical Greek thinking.
So, in dealing with the title of our opera, is it "It Must Be Fate" an appropriate title, given the subject matter is dealing with Classical Greek figures, the Moriai and thus should have some understanding in Classical Greek concepts? However, that said, perhaps the concept it must be fate, which is so much a part of our modern thinking (even though we tend to have the hubris to feel we are in control of our own destiny - which, if you think about it is an oxymoron, because, if it's your destiny, it's fated and therefore out of your control....) we do not fully realise where the concept came from. I said earlier it is a Christian concept, but how we perceive God (Jehovah) is based on concepts of Jove, the Roman Supreme God, who is based on Zeus. So, it is possible the concept of Fate (singular), God's intervention in our lives as something that is rooted in Classical Greek concepts, but not concept recorded in literature, because the concept was so accepted as part of society there was no need to elucidate it.
Thought and language is not a static entity. Concepts, which become woven into our society, are retained, if only as ghosts of their former selves, affecting future thoughts. A good example of this is "May the Force be with you." Not everyone in a modern society has seen the film Star Wars, but you'd be hard pressed to find someone who does not know what this statement means. In 100 years, we will likely still have the phrase. What concepts it will conjure may be vastly different than what we think today - but I have no doubt the concept of the force will play a role in our concept of God(s) and divinity for future generations. However, fifty years ago, this statement had no reference at all. Force was not used in this construct until the film (at least not in any literature I am aware of).
Maybe Zeus and Moriai were so much a part of the fabric of the Classical Greek culture they didn't need explanation. The thread of thought from then to now has shifted and changed. Saying something must be fate doesn't need further explanation, although not everyone would agree some event was indeed fated. Maybe, just maybe, it's time we remind ourselves of our past and how we got where we are. Maybe, just maybe, this is necessary as we look toward the future. Maybe, just maybe, this is fate.
έτσι το θέλει η μοίρα!