The Shaytaan whispers to him about who created Allaah

This
insinuating whisper will not have any effect on him. The Prophet
(peace and blessings of Allaah be upon him) told us that the Shaytaan
comes to a person and says to him “Who created such and such? Who created
such and such?” until he says, “Who created Allaah?” The Messenger of
Allaah (peace and blessings of
Allaah be upon him) has told us of the best medicine for dealing with
that, which is to seek refuge with Allaah from the accursed Shaytaan
and to put a stop to that. 

So
if any such thing occurs to you or crosses your mind, say “A’oodhu
Billaahi min al-Shaytaan ir-rajeem (I seek refuge with Allaah from
the accursed Shaytaan)” and give up this thought and ignore it completely.
Then it will go away by Allaah’s Leave.