Having Ill Thoughts about Allaah is one of the Major Sins and it negates the Perfection of Tawheed
Allaah (Ta’ala) says in Surat al-Fat-h (48:6):
الظانين بالله ظن السوء عليهم دائرة السوء وغضب الله عليهم ولعنهم وأعد لهم جهنم وساءت مصيرا
“…Those who think evil thoughts about Allaah, for them is a disgraceful torment. And Allaah has become angry with them and He has cursed them and prepared Hell for them, and worst indeed is that destination.”
Any person who believes that falsehood will prevail over Truth (i.e. Islam), and that it will cause Truth to disappear are thoughts of those who disbelieve. Whoever denies that things occur in accordance with Allaah’s Divine Decree and Decision or denies Allaah’s Supreme Wisdom behind His Decree, which is deserving of all praise, or thinks that things occur due to an aimless, purposeless Will, has thought ill about Allaah.
Many think ill of Allaah (Ta’ala) when it concerns that which affects them in particular, or what He inflicts others with. And none is saved from such evil thoughts except those who truly know Allaah and His Divine Names and Attributes.
If one looks closely at individuals, he will see many of them in distress and angry with what Allaah (Ta’ala) has decreed, finding fault and blaming Him for their affliction, thinking that it should have been such-and-such. In this, some are guiltier than others.
Examine yourself: Are you free from such thoughts? If you are safe from them, you have been saved from a great calamity, but if you are not, then you are not regarded as safe.
Muslims should never think ill of Allaah in any of the following ways or in any other possible way:
1- To think that Allaah is not above His Throne, apart from His creation, or thinking that He is everywhere.
2- To think that if one obeys Allaah, invokes Him and seeks His Aid and puts his trust in Him, that He will let him down and disappoint him.
3- To think that Allaah has begotten a son, or has taken a partner. To think that one can intercede with Him without His permission, or that between Him and His creatures are means or intercessors who raise the creatures’ requests and supplications. Or to think that He sets up guardians for His creatures to seek intercession through them.
4- To think that in Allaah’s Kingdom there is what He does not like or want, or what He is unable to make.
In conclusion: Thinking evil of Allaah is of numerous types that are unlimited, and they are all based on thinking of Allaah in a way that does not suit His Majesty.