He became Muslim then he left Islam
Question

                What is the Islamic ruling on someone who is a Muslim(who is in Islam) but converts to some other religion later on???
Praise be to Allah.
    Apostasy (riddah) means disbelieving after being Muslim. 
    There is no bad deed that cancels out all good deeds apart from apostasy. If 
    a person dies in a state of apostasy all his good deeds will be wiped out, 
    but if he returns to Islam, the reward for his good deeds will be restored 
    and he does not have to make up any prayers or fasts that he missed during 
    the period of his apostasy. 

    Shaykh al-Islam [Ibn Taymiyah] said: 

    With regard to apostasy from Islam, whereby a person becomes 
    a kaafir, whether a mushrik or a Jew or Christian, if he dies in that state 
    then all his good deeds will be wiped out, according to scholarly consensus, 
    as the Qur’aan says in more than one place. For example Allaah says 
    (interpretation of the meaning): 

    “And whosoever of you turns back from his religion and 
    dies as a disbeliever, then his deeds will be lost in this life and in the 
    Hereafter”

    [al-Baqarah 2:217] 

    “And 
    whosoever disbelieves in Faith, [i.e. in the Oneness of Allaah and in all 
    the other Articles of Faith i.e. His (Allaah’s) Angels, His Holy Books, His 
    Messengers, the Day of Resurrection and Al-Qadar (Divine Preordainments)], 
    then fruitless is his work”

    [al-Maa’idah 5:5] 

    “But if they had joined in worship others with Allaah, all 
    that they used to do would have been of no benefit to them”

    [al-An’aam 6:88]

    Majmoo’ al-Fataawa, 4/257, 268) 
With regard to the shar’i 
    rulings on apostates, if the apostate does not return to Islam, he must be 
    executed. 

    It was narrated that ‘Abd-Allaah ibn Mas’ood said: the 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    “It is not permissible to shed the blood of a Muslim who bears witness that 
    there is no god except Allaah and that I am the Messenger of Allaah except 
    in one of three cases: a soul for a soul (i.e., punishment for murder), a 
    married man or woman who commits adultery, and one who leaves his religion 
    and separates from the jamaa’ah (main body of Muslims).” 

    Narrated by al-Bukhaari, 6484; Muslim, 1676 

    It was narrated that Ibn ‘Abbaas said: the Messenger of 
    Allaah  (peace and blessings of Allaah be upon him) said: “Whoever 
    changes his religion, execute him.”  

    (Narrated by al-Bukhaari, 6922). 

    Then if he is executed, he has died as a kaafir, so he is not 
    to be washed or shrouded; the funeral prayer is not to be offered for him, 
    and he is not to be buried in the Muslim graveyard. On the Day of 
    Resurrection he will be one of the people of Hell who will abide therein for 
    eternity. 

    The earth rejected the body of an apostate during the time of 
    the Prophet  (peace and blessings of Allaah be upon him), as a lesson 
    and a warning to the onlookers. 

    It was narrated that Anas (may Allaah be pleased with him) 
    said: There was a Christian man who became Muslim. He used to recite 
    al-Baqarah and Aal ‘Imraan, and he used to write down (the revelation) for 
    the Prophet  (peace and blessings of Allaah be upon him). Then he 
    returned to Christianity and he used to say, “Muhammad knows nothing but 
    what I have written for him.” Then Allah caused him to die, and the people 
    buried him, but in the morning they saw that the earth had thrown his body 
    out. They said, “This is the doing of Muhammad and his companions. They dug 
    the grave of our companion and took his body out of it because he had run 
    away 

    from them.” They again dug the grave deeply for him, but in 
    the morning they again saw that the earth had thrown his body out. They 
    said, “This is the doing of Muhammad and his companions. They dug the grave 
    of our companion and threw his body outside it, for he had run away from 
    them.” They dug the grave for him as deep as they could, but in the morning 
    they again saw that the earth had thrown his body out. So they realized that 
    what had befallen him was not done by human beings and they had to leave him 
    thrown (on the ground). 

    Narrated by al-Bukhaari, 3421; Muslim, 2781. 

     And Allaah knows best.