Hello my friend:
I see your point of view and appreciate your thoughts. Humans can call God whatever they want but that does not change reality. God is absolute one and that the only thing that makes sense.
Now going to very important point which you made is that most religions started very late in human history. It is true in most cases but you need to understand one religion in particular which is Islam. If you think that is the religion started by Prophet Muhammad PBUH some 1400 years ago then you are mistaken.
Islam literally means submission to God. Quran tell us that this was the religion of very first human being Adam and was the religion of all prophets including Noah, Ibrahim, Moses, Jesus and many other prophets. Muhammad PBUH is the last prophet because he came with final guidance for entire humanity which is in the form of Quran. SO to sum up, the religion for entire humanity from the beginning has been one and that is Islam.
Guidance has comes to different nations and tribes through different prophets and it was always for a particular group, it never was sent for entire humanity. Even Jesus was sent to fulfill the law of Moses which was for people of Bani Israel (Jews).
If you read Quran you will see many common theme between OT and Quran which are thrown out in NT. Quran has revived those and updated several laws to fit the needs of entire humanity till the day of judgement.
As I mentioned in some of my other posts that the purpose of this life is a test. So there will be a battle between good and evil at all times. There will be suffering and people were/are/will be killing each other. Some of those will be for right reasons such as for self protection and others will be for wrong reasons such as oppression and power gain. That's is a common theme throughout human history. It is therefore important to know what is good and what is bad. Not by our standards but by the guidance given by God.
For example, for some having sexual relation outside the wedding lock may be fine but is it fine in divine law? It may be a major sin which we may be committing without knowing it.
From a common day example, if you buy a new phone and it happen to have some problems then how do you fix it? Call customer service or looking in the manual right? these manuals are religious books given by God and customer service representative as similar to prophets.