What is disbelief?

 If someone denies even a single matter that is certainly known to be part of Islam, it is considered disbelief (kufr). For example, denying the Day of Judgment, angels, Paradise, Hell, or accountability in the Hereafter; rejecting the obligation of prayer (Salah), fasting (Sawm), pilgrimage (Hajj), or almsgiving (Zakat); not believing that the Quran is the word of Allah (عَزَّوَجَلَّ); disrespecting the Kaaba, the Quran, any prophet, or any angel; belittling any Sunnah; mocking the rulings of Shariah; or denying or doubting any well-known and established aspect of Islam—these all constitute kufr.

To be a Muslim, both faith (Iman) and verbal declaration (Iqrar) are necessary, except in situations of compulsion. If a person is unable to speak due to a disability, if uttering words of faith would result in death, or if a limb would be severed, then verbal declaration is not obligatory. In such cases, even speaking words contrary to Islam to save one’s life is allowed, but it is better and more rewarding to remain silent.

However, outside of such extreme circumstances, if a person utters words of disbelief, they will be considered a disbeliever (kafir), even if they claim that they only said it with their tongue and not from the heart. Similarly, if a person engages in actions that are recognized as signs of disbelief—such as wearing a sacred thread (janeyu), keeping a particular religious hairstyle (choti), or wearing a cross—they will be deemed a disbeliever.