The central belief of Islam is the oneness of God, Allah (see What is Islam?). The Quran teaches that God is known to His creation through His "most beautiful names" (al-asma al-husna) — names that describe His perfect attributes.567
Calling on God by His names
"And to Allah belong the best names, so invoke Him by them."
— Quran 7:180 (Saheeh International)1
"Allah — there is no deity except Him. To Him belong the best names."
— Quran 20:8 (Saheeh International)3
What the names describe
Several of the names appear together in one passage, giving a sense of how they portray God:
"He is Allah, other than whom there is no deity, Knower of the unseen and the witnessed. He is the Entirely Merciful, the Especially Merciful. He is Allah … the Sovereign, the Pure, … the Exalted in Might … He is Allah, the Creator, the Inventor, the Fashioner; to Him belong the best names."
— Quran 59:22-24 (Saheeh International)2
The two names invoked most often are Ar-Rahman and Ar-Rahim — "the Entirely Merciful, the Especially Merciful" — with which Muslims begin nearly every chapter of the Quran and many daily actions.
The ninety-nine names
Islamic tradition speaks of ninety-nine such names. The Prophet Muhammad ﷺ said:
"Allah has ninety-nine names, one hundred minus one; whoever enumerates them will enter Paradise."
— Sahih al-Bukhari 27364
These names — among them the Merciful, the All-Knowing, the All-Powerful, the Forgiving, the Just, and the Wise — are a primary way Muslims come to know and remember God.
In short
Muslims know the one God through His most beautiful names, traditionally counted as ninety-nine, which reveal His attributes of mercy, power, knowledge, and wisdom. To call on God by these names is itself an act of worship.