Praise be to Allah:
The causes of the decline of faith as follows:
The first cause: ignore the names and divine attributes would result in the faith for he who feels a lack of knowledge about the names and Allah attributes undergoes a decrease in
her faith.
The second cause: turn away from reflection on the natural signs and religion of Allah. This attitude causes otherwise decreased at least his faith stagnation. The third cause: disobedience; she has very important effects on the heart and
on faith. That is why the Prophet (Peace and salvation be upon him) said:
“The fornicator does not commit his act is a believer … “.
The fourth: the abandonment of obedience. This abandonment causes decreased faith. If the act is an obligation abandoned and if the abandonment is not based on excuse, it drives said reduction and author will be blamed and punished. However, if abandoned the act is not an obligation and if the abandonment is excusable one will blame not its author for the decrease that results. (…).
(Source: Majmoo Fataawa wa rassaïl by Shaykh Muhammad Ibn Salih al-Uthaymeen, Volume 1 p. 1052)