One of the greatest reasons for which Allah has created us – which is one of the greatest tests – is the command to affirm His Oneness (Tawhid) and to worship Him Alone with no partner or associate. For more, see the detailed answer.

Faith in Allah means believing firmly in His existence, Lordship and Divinity, and in His Names and Attributes. There are four things to be avoided, whoever falls into one of them has not attained true belief in the names and attributes of Allah, These four things are: Tahreef (distorting), Ta’teel (denying), Tamtheel (likening Allah to His creation), Takyeef (discussing how).

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.

“la ilaha illa Allah” means nothing worshipped is worthy of worship except Allah. “Muhammadun Rasul Allah” is the assertion of the message of Prophet Muhammad and to believe in it and to adhere to it by speech, action and faith.