I remember my aunt's husband saying something years back; I think he said something close to ‘Has allaah gone blind or something?’ or something similar. Also, my parents were having a discussion about the rain, and one of them ended up saying something like 'Call up Allaah [with a phone/mobile] and tell Him to stop it from raining.' These situations and people that I mentioned, from what I know, they call themselves Muslims, and they pray. Should I doubt their faith? Have they committed an act of disbelief? Please provide a very detailed answer.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

As for the first statement, you should not doubt or accuse the person who said it, because it is possible that it was a rhetorical denial question intended to confirm that Allah sees what is happening.

As regards the statement of the parents, it is wrong, and it is an ugly thing to say, but we cannot rule that the one who said it is a disbeliever because it is possible that he was ignorant of the fact that saying that is wrong.

A Muslim is not accused of disbelief unless he fulfills the conditions for declaring him a disbeliever while there are no impediments which prevent from declaring him as such; among which is that he should be an adult, in sound mind, intending what he did or said, doing it willingly, and not excused for ignorance or misinterpretation in matters where ignorance and interpretation are a possible excuse.

Ibn Taymiyyah said, “No one should declare a Muslim a non-Muslim, even if such a Muslim has done wrong and erred, until the evidence is established against him and the truth is clarified to him. Verily, he whose Islam is established with certainty cannot be declared a disbeliever because of doubt (it can only be done with certainty). Rather, he cannot be called a non-Muslim except after establishing the evidence against him and removing all misconceptions that he might have.”

He also said, “There are terms and conditions for declaring someone a disbeliever, and there are impediments that prevent from declaring him as such which may not exist in a particular person. General classification of a word or deed as disbelief does not necessarily entail accusing a person who says or does it of disbelief unless all the conditions of Takfeer (declaring someone a disbeliever) are met in his case while there are no impediments...”

‘Ali Al-Qaari said in Sharh Ash-Shifa:

“Our scholars said, ‘If there are ninety-nine aspects indicating that a Muslim has become a disbeliever and there is only one aspect indicating that he is still a Muslim, then the Mufti and the judge should act according to this one aspect. This is deduced from the saying of the Prophet 'Avert the legal penalties from the Muslims as much as you can; if you find a way out for the Muslim, then let him be (do not punish him), for if the Imaam makes a mistake by forgiving, it would be better for him than making a mistake by punishing.'’” [At-Tirmithi and Al-Haakim]

For more benefit, please refer to fatwas 15255 and 343903.

Allah knows best.