In principle, an atheist may feel smug by countering the supposed theistic assertion that: ‘Everything must have a cause for its existence’, with: ‘So what caused God?!’
In practice, no Muslim theologian (nor any Jewish or Christian one) has ever asserted this. Rather the theistic belief is: ‘Everything that comes into existence, from non-existence, must have a cause for its existence.’ God, however, did not ‘come into existence’. He necessarily exists. God’s eternal attribute of life is intrinsic to, and inseparable from, His holy Essence.