Sufi,
I believe you have the question reversed. Allah is entirely defined by Islam, so to know what is meant by the concept of Allah one must study Islam. In an identical manner one cannot fully comprehend the nature of Sherlock Holmes without reading and studying the works by Sir Arthur Conan Doyle.
Is it possible to truly understand Islam without knowing what 'Allah' is?
I believe you have the question reversed. Allah is entirely defined by Islam, so to know what is meant by the concept of Allah one must study Islam. In an identical manner one cannot fully comprehend the nature of Sherlock Holmes without reading and studying the works by Sir Arthur Conan Doyle.