Is it permissible, in Islamic law, to write a letter to God?

There is no objection to metaphorically calling praising Allah and supplicating to Him a "message," for Allah is Near and All-Hearing. Allah Almighty says: "And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he invokes Me." And in the Hadith: "Indeed, you are not supplicating one who is deaf or absent; you are supplicating one Who is All-Hearing, All-Seeing, and Near."