assalamu alaikum what happens if a muslim lies & says he's not a muslim? & what happens if he lies about another muslim saying he's christian?

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

It is not permissible for a Muslim to announce willingly and without being compelled that he is a non-Muslim; the jurists stated that if a person admits he is a non-Muslim, then he goes out of the fold of Islam unless he has a sound reason for doing so, like being compelled.

The book Fat’h Al-‘Aliyy Al-Maalik authored by Shaykh ‘Ulaysh while clarifying the criterion by which a person goes out of the fold of Islam, reads: “Ibn Rushd said: “A person is not ruled to be a non-Muslim except with the following three matters: two matters of them are agreed upon, while there is a difference of opinion about the third one. As regards the two matters that are agreed upon, one of them is that he admits that he does not believe in Allaah…”

However, it makes no difference that he does so jokingly or seriously; as joking is not permissible in such a case. Allaah Says (what means): {And if you ask them, they will surely say, “We were only conversing and playing.” Say, “Is it Allaah and His verses and His Messenger that you were mocking?” Make no excuse; you have disbelieved [i.e. rejected faith] after your belief. If We pardon one faction of you—We will punish another faction because they were criminals.} [Quran 9:65-66]

Moreover, some scholars stated that if a person claims to be a non-Muslim, he becomes an apostate even if he is joking, except in case he is making an oath. Ad-Dardeer in his commentary on Sharh Khaleel (a book in the Maaliki School of jurisprudence) said: “…Or that he says that he is a Jew or a Christian, or an apostate, or says that he belongs to a religion other than Islam if he did such and such a thing and he actually did it, then he is not required to do anything, but it is forbidden for him to do so, but if he did it in other than an oath, then it is an apostasy even if he was joking.” [End quote]

On the other hand, declaring a Muslim as a non-Muslim is one of the grave major sins. Doing so without evidence is very serious; indeed, the Prophet said: ''If a man says to his brother (i.e. a Muslim person) O Kaafir (non-Muslim), then it returns upon one of them.” [Al-Bukhari and Muslim]

The Prophet also said: “Whoever calls a man (a Muslim person) a Kaafir or says to him ‘O enemy of Allaah’, and he is not such, but that it returns back to him.” [Al-Bukhari and Muslim]

The threat in this Hadeeth is for warning and highlighting the seriousness of this act without meaning the disbelief of the person who said so, as Ibn Qudaamah said in his book entitled Al-Mughni: “These Ahaadeeth are to show the seriousness of this act and to repel people from saying so, and to resemble someone who said this to the non-Muslims, and it is not really meant that a person who said so becomes a non-Muslim.” Nonetheless, this threat is evidence for the seriousness of uttering such an expression and that it is an obligation to preserve one’s tongue (i.e. to avoid uttering such terms) as this may lead the one who said it to destruction. For more benefit, please refer to Fataawa 91677 and 84943.

Allaah Knows best.