Hmm... "What is written" is the present tense in the passive? Right? So, shouldn't be: "Quod scribitur"? I know it is confusing indeed, but if it was "What was written", we would translate "Quod scriptum est"... Am I right, dear Alex?
Yes, you're absolutely right, but I considered "it is written" not as a passive voice of the verb "to write", but as the verb to be plus the participle written acting like an adjective. Can you see what I mean?
I do not know if we can translate it literally to Latin...