"(3) His being is not merely One (wahid but ahad, in which there is no tinge of plurality in any way: He is not a compound being, which may be analysable or divisible. which may have a form and shape, which may be residing somewhere, or may contain or include something, which may have a colour, which may have some limbs, which may have a direction, and which may be variable or changeable in any way. Free from every kind of plurality He alone is a Being Who is Ahad in every aspect. (Here, one should fully understand that the word wahid is used in Arabic just like the word "one" in English. A collection consisting of great pluralities is collectively called wahid or one, as one man, one nation, one country, one world, even one universe, and every separate part of a collection is also called one. But the word Ahad is not used for anyone except Allah. That is why wherever in the Qur'an the word wahid has been used for Allah, He has been called Itah wahid (one Deity), or Allah-ulWahid-al-Qahhar. (One Allah Who is Omnipotent), and nowhere just wahid, for this word is also used for the things which contain pluralities of different kinds in their being. On the contrary, for Allah and only for Allah the word Ahad has been used absolutely, for He alone is the Being Who exists without any plurality in any way, Whose Oneness is perfect in every way. (Tafheemul-Quran)" (source; emphasis mine)
It is on this basis that Muslims strenuously object to the doctrine of the Trinity, the doctrine that the Father, the Son, and the Holy Spirit are one in essence but still personally distinct. However, if Muslims were consistent with their own definition of "pure monotheism", then they would not go on to speak of Allah having 99 beautiful names and many attributes, for this is to assert a kind of pluarlity in unity that is contrary to their proferred definition.
So which is it, do Muslims believe in the "names" and "attributes" of Allah, or is Allah an undefinable blank, a mere abstraction?
Either choice spells trouble for the followers of Muhammad.