What is the ruling if a person unintentionally or for joke says he want to enter Christianity or he wanted to become Jew or wanted to enter other religion than Islam...does this make him a kafir straight away?what we have to do if we make mistakes and unintentionally utter such words...thank you.

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

It is not permissible for the Muslim to attribute himself to a religion other than Islam even when joking. This is a very serious matter. Apparently – and Allaah Knows best – a Muslim does not become a disbeliever merely by saying that he wants to enter Christianity or the like.

However, if he says so while approving this religion, he will become a disbeliever since wishing for disbelief is in itself disbelief. Muslim jurists stated that a Muslim who swears that he is a Jew or a Christian should he does such and such a thing, he does not become a disbeliever even if he did what he said in his oath so long as he does not inwardly profess disbelief. In “Al-Mudawwanah”, a man asked ‘Abdur-Rahmaan ibn Al-Qaasim : "If a man says that he would become a disbeliever if he did a particular thing, is this considered an oath according to the opinion of Maalik?" He said: "Maalik said that this would not be an oath, nor would the man become a disbeliever unless he professes disbelief in his heart, and how evil his action is!"

Therefore, a Muslim who committed any of these things has to sincerely repent to Allaah The Almighty of it and determine not to repeat it in the future.

Allaah Knows best.