Muslims say that God can be known because the Qur’an is the Word of God:
The revelation of the Book [this Qur’an] is from Allah [swt], the All-Mighty, the All-Wise.
Therefore, Muslims know that God speaks.
The Qur’an, in turn, reveals what God is like, for example,
He [Allah (swt)] begets not, nor was He begotten.
He is Allah, the Creator, the Inventor of all things, the Bestower of forms. To Him belong the Best Names. All that is in the heavens and the earth glorify Him. And He is the All-Mighty, the All-Wise.
Say [O Muhammad (saws)], “He is Allah, [the] One.”