Islam is a really bad example, since it's an extremely late entry in the whole history of God, as such it is not a good basis to study the God concept at all. If you want to scientifically validate the existence of an eternal God it makes a lot of sense to go for the oldest sources, and the oldest sources are the pentateugh (the first 5 books of the bible). There is precious little evidence the quran as a book even existed before the 19th century (some stories can be traced back to ~800 AD, but they are copies of pages of the Arameic bible, they are not unique to the quran. Also the national library of Turkey and some organisation in Baghdad claim to have older qurans but they won't show them to anyone. Several articles by an ex-keeper of that book in Turkey indicate that this is because there are significant differences). Despite how arabic script looks, arabic is actually not that far removed from greek (as in there exists a literal transliteration between arabic and greek that reduces the language difference to vocabulary, more or less). The older qurans aren't even in Arabic, they're kufic (which is a dialect of Arameic, and "just happens" to be the dialect a significant part of the bible is written in. Long story short : the quran is an especially badly made copy of the Arameic new testament with significant amended by a successful warlord with Jewish heritage)

But if you look at real early texts you will find no mention of God. So how do you arrive from the early books to God ? Well, easy : anything expressed in one of the passive tenses refers to God(s) (except where it really doesn't make sense). Meaning in the really old portions of the bible (we actually have old versions of the bible, in contrast to the quran), you will not find a sentence like "God made it rain", instead you will find "It was raining" and that is currently getting translated using God. There are other factors that make matters even more confusing.

Even in competing religions' texts you will find this. The Greek religions for example also do this. Until the late Roman republic you almost never find any reference to direct action of any of the Greek/Roman Gods. Rather you will find references that look a lot like today's "lady luck was smiling on him". In stories you will find that, for example, a king "felt inspired by Ares", which I'm sure you can translate for yourself. Or "Ares smiled on the king's forces", indicating he won (note that Greek/Roman Gods were not really part of a pantheon. They are more like "patron saints" of specific families or other groups of people. For example, when there's a battle between Sparta and Athens, either Ares "smiles" or Athena does, meaning this indicates which side won (extremely confusing the first time you read that. You read a story of a battle, with lots of descriptions of fights between generals, heroes, etc. Then suddenly "Ares smiled" and it starts describing logistics of returning the army to the Greek mainland. First time you'll be sitting there "wait, wasn't there a battle ? How did it end ?), and Greek/Roman gods were probably understood to be the personality of a particular city state (or states). The pantheon in Rome is simply meant to indicate an alliance between specific families/states).