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.
If writing such words that indicate that the person apostated from Islam and embraced Christianity is accompanied with the uttering of such a word, then the person who does so becomes a non-Muslim and goes out of the fold of Islam.
The scholars stated that uttering the word of disbelief takes the person out of the fold of Islam whether the person is joking or is serious.
Ibn Nujaim said: “Whoever says the word of disbelief jokingly, he becomes a non-Muslim according to all scholars and it does not matter what his intention was.”
Moreover, Imaam Ibn Taymiyyah said: “Uttering a word of disbelief voluntarily certainly reflects that the one who said it is content with disbelief.”
However, if this was limited to only writing the word without any intention and without being content with such words that he has written, then it appears that this does not reach the state of apostating (going out of the fold of Islam). However, the one who says it is in great danger. Allaah may become angry with him, and thereby seal his heart, and he could consequently fall into real disbelief. Thus, a Muslim has to be very careful not to lose his religion for a meaningless temporary worldly benefit because this actually means losing his worldly life and that of the Hereafter.
Anyone who helps someone to do this is sinful because Allaah Says (what means): {Help you one another in virtue, righteousness and piety but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment.} [Quran 5:2]
Allaah Knows best.