If the truth is that the first sentence is the most important, then this sentence is the second one.

A beautiful beginning, isn't it?
Truly beautiful. And what next?
That's right: what next?
Of course, it's all about the first sentence in a novel. No doubt. Or maybe also in a short story. Or in an essay. Or in any literary text written in prose. Surely not in a vacuum cleaner manual. But why not? Does a vacuum cleaner manual, or any device manual, has to be boring?
If you bought this vacuum cleaner, don't expect your entire life will now be cleaner in all of its aspects, and if something like that seems to you, please take note of great disappointment waiting for you, because this device will not suck into its bag all the dirts, especially those non-material incorporeal ones.
The most important means “which is on the top”. This is one of the possible and common images of themostimportance. So, the first sentence, as the most important, would be on the top of a mountain of sentences a book is. Or a hill, a hillock, a short story is. Simply – the peak. According to this image, and there are many others, all next sentences, as less important, would be placed beneath. The less important sentence, the lower it is. Lower meaning closer to the end. And the least important would be the last one. The last sentence would be absolutely insignificant. So unimportant would it be, that it could be not existing at all.
Let's delete it.
It has been deleted.
If we deleted it, if now our novel or short story has no last sentence, then, most probably, it has the second to last sentence, or the one which was second to last when the last one had been. And now this second to last sentence has become the last one. Although it was a little bit more important than the deleted unimportant one, now it is the unimportant one, because it is the last one. Of course we mean the relative importance, the importance defined by the relation with the first sentence, not the absolute importance – the absolute importance doesn't interest us for it would disturb the disorder of thoughts, and now we have to delete the last sentence (which was the second to last sentence a while ago) as insignificant comparing to the incredible significance of the first sentence (which in fact is the former second sentence).

We delete it.
OK. It's out. Off and away.
Instead there is a sentence which once was third to last, and now is the last one.
And so on. Till we get to the second sentence which once had been the third one, before we deleted the first sentence… Preferably before we announced the lack of the first sentence… But why should the first sentence be lacking?… Now the second sentence is the last one, since only two sentences have left. And now the last sentence should be deleted. Erased, covered, smudged. We would have only one sentence. This sentence would be the first one. The most important one. Thus our book, our novel or short story, would be composed of only the most important sentences. It would be a masterpiece, because masterpieces are just such works, they are works consisting of only the most important sentences. Would this mean that masterpieces of literature are the one-word works? That only one-word works could be masterpieces of literature? This is a supposition. Another supposition, much closer to a statement, is the one saying that such books will be flawless, because there will be no less important sentences in them let alone the absolutely unimportant and insignificant ones (provided that a sentence is flawless too).
It would be so, it could be so, if we perceived a book as a mountain of sentences. A mountain with a sharp peak. It would be so, if we perceived a book as a cone. Or as a pole. Even as a stick dug vertically into the ground. Or if a book were a butte on a vast flatland. It shouldn't be forgotten, that in case of one sentence, in case of one-sentence book, the first sentence is the last one. Thus it would be the most important and absolutely insignificant in the same time – this could take us to the very attractive concept of a no-sentence book. Attractive for whom: for a writer or for a reader or for both? With no doubt for an editor and a proof-reader – they wouldn't complain about the excess of really boring work.
And if a book were like a vast plain? What would be then? Which sentence would be the first one?
A book like a king with no kingdom.
A book like a kingdom without a king.
A peak without a mountain.
A mountain with no top.
And so on. So many variants and versions in between. The abundance making you dizzy.

What has been written above seems a pure nonsense. However, more careful reading can make us change this opinion and we will consider this nonsense dirty and impure, and notice an ugly patch covering the prefix non.
Why would the first sentence be so important that even the most important? Because something begins with it? Because it is a kind of door opening?
First sentences can be different. Some of them are so banal; if they were to announce the whole text, next sentences, we should stop reading immediately. What can one expect from It was summer ? Everything and nothing. And now it turns out the most banal sentences, neither important nor unimportant in both relative and absolute ways, so simple and typical sentence is a great one, really great, because it promises nothing, imposes nothing, defines nothing, takes you into no direction, gives you a possibility to go anywhere. It can happen that a banal beginning takes us to the fascinating plot and delighting end, while an awesome beginning turns out to be the only interesting sentence, as if it exhausted the writer completely. Different things can happen.
Let's think also about the beginning – what is it? Why the first sentence is supposed to be the beginning? And if it were, then the beginning of what? Do stories and histories have the beginnings and ends? Beginnings and ends are nothing but illusions, premises helping us to move around the labyrinth of the world, but they make this labyrinth even more complex and tangled. These are the default points, points of reference. That is why the lack of the first and the last sentences has the profound sense, because both beginning and ends are unclear, smudged, misty, undefined... All these considerations are based on a premise that a reader starts to read from the very first sentence and the very first page, that this is his first contact with the written story. It's so different in so many cases, indeed. We take a book, a new book, we open it randomly and read a sentence just caught by our eyes. For example the sentence number one thousand six hundred twenty eight. And it can happen this very sentence would be a hole in the fence surrounding the garden of this book. We take a look inside. Then we go looking for another hole. Only after that we will try to find a gate. And then? . . . Then the sentence number one thousand six hundred twenty eight must be the most important. Like any other sentence. It should be so. But it isn't so. Sometimes it is so. It can happen every sentence is a gorgeous meadow in full bloom. It happens so. Rarely. Very rarely So rarely that almost never. While it should be so. It could be so.