Waswas comes from three sources: the nafs (or self), which is inclined to evil, the devils among the jinn (demons), and the devils among mankind.

To get rid of bad thoughts about Allah, you should say, “Amantu Billahi wa Rasulihi (I believe in Allah and His Messenger).” Try to stop thinking about that as much as possible, and keep busy with things that will distract you from it.