What is the ruling regarding someone who assumed a person was not Muslim, and then it became clear to him that the opposite was true?

It is forbidden to harbor ill thoughts about a Muslim if they are in Muslim lands, for a believer is held in greater sanctity by Allah than the Kaaba. The Prophet (peace be upon him) said: "Indeed, Allah has forbidden one thing concerning you (the Kaaba), and forbidden three things concerning the believer: his blood, his wealth, and to harbor ill thoughts about him."

However, if the Muslim is in infidel lands, one does not sin by considering them a disbeliever. This is based on the action of the Prophet (peace be upon him) when he would raid people after morning if he did not hear the call to prayer, despite the possibility of their conversion to Islam, and the general principle is that the inhabitants of the land of disbelief are disbelievers. And based on the Prophet's (peace be upon him) statement: "I am innocent of any Muslim who resides among the polytheists," because they brought it upon themselves by residing among disbelievers.

The verse: "O you who have believed, when you go forth [to fight] in the cause of Allah, investigate, and do not say to one who offers you peace, 'You are not a believer,'" indicates that it is impermissible to harbor ill thoughts about someone who displays signs of Islam.

As for a Muslim who is in a Muslim country but shows signs of depravity, some scholars have held that it is permissible to harbor ill thoughts about them. The ill thought that is forbidden is that which has no basis. However, if a person is known for engaging in suspicious activities and openly committing evil deeds, there is no harm in harboring ill thoughts about them. An example of this is the Prophet's (peace be upon him) suspicion of the hypocrites that they knew nothing of the religion.

It has been established among scholars that whoever abandons fasting in Ramadan without a valid excuse is worse than an adulterer and an alcoholic, and their Islam is doubted, and they are suspected of apostasy and moral decay.