On the wiki, it is mentioned that the Greybeards consider the very existence of the Elder Scrolls to be a blasphemy.
Why is this?
According to the wikipedia article The Elder Scrolls are by nature unstable:
The actual number and placement of elder scrolls fluctuates constantly as it is said that they technically exist and do not exist at the same time. This makes their predictions difficult to cite authoritatively because entire scrolls or entries can change or vanish as events transpire. This unpredictability has caused other ascetic groups, such as the Greybeards from Skyrim, to find the existence of the Elder Scrolls a blasphemy.
The wiki article you point to specifically cites this line of dialog from Arngeir (in response to the Last Dragonborn asking for help in locating a Scroll):
We have never concerned ourselves with the Scrolls. The gods themselves would rightly fear to tamper with such things. As for where to find it... such blasphemies have always been the stock in trade of the mages of Winterhold. They may be able to tell you something about the Elder Scroll you seek.
For anyone unfamiliar with the metaphysics of the Elder Scrolls universe, this may come as a rather surprising line of dialog. But I can state with confidence that this is not hyperbole or exaggeration; Arngeir is being completely literal when he says that the "gods" would fear the Scrolls. So, here's an extremely abbreviated primer on what "gods" Arngeir is referring to, and why they might fear the Scrolls:
To begin, we should acknowledge that this is an enormously broad subject. There are well over two hundred in-universe books about religion, prophecy, and related topics. I will be sketching out the bare minimum which I think is absolutely relevant to this topic, and simplifying as aggressively as possible.
Deities in the world of the Elder Scrolls can be broadly divided into Aedra and Daedra (Aedra and Daedra), who both coalesced from the interplay of Anu and Padomay at the beginning of the world (The Monomyth). This is not an exhaustive categorization (Anu and Padomay themselves are neither Aedra nor Daedra, and there are several other categories of being in the Elder Scrolls mythology), but it's good enough for the time being.
The Daedra comprise everything from a lowly atronach all the way up to the various Daedric Princes who appear throughout Skyrim and the earlier games. Most Daedra are subordinate to one of those Princes (for example, the Dremora are generally subservient to Mehrunes Dagon), but very low-level Daedra such as atronachs are unaligned. For the most part, Daedra tend to hang out in whichever Plane of Oblivion their master rules over. Daedra are eternal and effectively immortal, because they are not bound up in the mortal world, so the most you can do to them is banish them back to Oblivion. However, quite a lot of mythology describes their interactions with and influences on one another (see e.g. The Fall of Trinimac), so it seems that eternity is rather more flexible on this point than one might expect.
The Aedra, on the other hand, are comprised of the Eight Divines (Talos's exclusion from the Divines is Aldmeri propaganda at best, but calling him an Aedra is not precisely correct; I'm also omitting Magnus, the Magna Ge, and the Ehlnofey because they are not relevant to the question). Together with Lorkhan (also known as Shor, Shezarr, and a number of other names besides), they are the creators of Mundus or Nirn (the mortal plane). Opinions vary widely on whether Lorkhan tricked or honestly convinced the other Aedra into this act, and whether doing so was a good thing (opinion of the human races) or a bad thing (opinion of the Aldmeri Dominion), but he is generally regarded as instrumental to the event (Before the Ages of Man). Regardless, everyone mostly agrees with the following:
In summary: Arngeir is almost certainly referring to the Eight or Nine Divines when he says "the gods themselves," rather than the Daedra or some other set of beings. The Divines are mortal beings of limited power, and significantly weaker than the Daedra. Although they created Mundus, they did not create the universe as a whole.
So now that we know what group of beings Arngeir is talking about, the next step is to figure out why they would fear the Elder Scrolls.
The short and disappointing answer to this question is that nobody (in-universe) seems to know what the Elder Scrolls are, where they came from, or exactly how they work. Paarthurnax calls an Elder Scroll "an artifact from outside time. It does not exist, but it has always existed," which implies that they pre-date the Aedra and Daedra, and Ruminations on the Elder Scrolls provides a series of amazingly unhelpful analogies to further "explain" the nature of the Scrolls (but Septimus Signus may not have been entirely sane when he wrote it). See also Effects of the Elder Scrolls for some general discussion on the "how they work" point. However, Paarthurnax does give us this somewhat helpful dialog:
Player: "How could an Elder Scroll cast Alduin through time?"
Paarthurnax: "Vomindok. I do not know. Perhaps in the very doing they erased the knowing of it from Time itself. The dov are children of Akatosh. Thus we are specially… attuned to the flow of Time. Perhaps also uniquely vulnerable. I warned them against such a rash action. Even I could not foresee its consequences. Nust ni hon. They would not listen."
This strongly implies that the dragons inherited some measure of vulnerability from Akatosh, who is generally regarded as the patron deity of the Divines (Varieties of Faith in the Empire). Combined with the fact that the Aedra are mortal, and can be killed, it seems reasonable to characterize the Scrolls as, in general terms, more powerful than the Aedra, and possibly even a threat to them. Whether they are more powerful than the Daedra is less clear to me, but they are certainly objects of great significance regardless.
Throughout Skyrim, the Greybeards repeatedly demonstrate a profound distrust in power and strength for its own sake, and only very reluctantly assist the Last Dragonborn in fulfilling the Alduin prophecy. This is an explicit and very important component of their core philosophy. At one point, Arngeir even suggests the Dragonborn allow the world to end. It really should not be too surprising that they would regard an object more powerful than the gods themselves as "blasphemous."