The letter was published on Al-Ittihad's website, a daily newspaper that covers Iraqi politics.
Here is the translation of the letter:
The General Secretariat of the Adherent of Islam Brigade has decided to address the final warning to you and to all your adherents and flocks, the infidel Christian Crusaders, in Baghdad and the other governorates and order you to leave immediately, in masses and permanently from the Muslim countries (Iraq) and to join Pope Benedict the sixteenth and his followers who crossed over the greatest symbols of humanity and Islam.
There is no place for you infidel Christians among the Muslim believers in Iraq from now on. Otherwise, our swords will be legalized over your neck and the necks of you followers and flocks similar to what happened to the Christians living in Mosul.
God is our witness
He who warns is excused
The General Secretariat of the Adherents of Islam Brigade