The dictionary definition of 'Conspiracy' is:
a secret plan by a group to do something unlawful or harmful.
To me, this doesn't apply to the majority of religions and religious people.
'God' wasn't 'invented' at all really. It is an idea which has evolved over thousands of years.
Originally, tribal groups thought that the animals and trees contained spirits.
They didn't have a concept of a 'God' as we recognize that word today.
Eventually, these groups advanced into ancestor worship in which they believed the spirits of their ancestors lived on after death.
They still had no concept of God at this point, but the foundations of polytheism were laid here when the idea of 'luck' sprang up. Some spirits were viewed as more powerful than others and would do favors for humans if they were given sacrifices.
That thought process eventually led to polytheism, like the Greek pantheon, which eventually led to philosophers like Plato suggesting that one singular God created everything.
He called it the 'Demiurge' but he didn't suggest this because he wanted to influence the world into following a made up deity.
He actually stumbled upon the idea because he thought it was the most logical conclusion to one of his other theories.
It's actually quite incredible that one of the first mentions of a single God was due to a theory which is now unknown to most people who believe in God.
Plato believed that for every single item in existence there was a perfect form of it in a higher level of existence. There would be a perfect table, on which every table in existence is based, for example.
It's a bit of a silly theory, but of course, to Plato's thinking, it would mean that there would need to be a form for things like beauty, love, etc. too.
Then there would need to be a form of all of the forms, Plato concluded that it would need to be a God-like figure - close to how we think of it today.
In his cave allegory, the sun plays the role of the ultimate form, which Plato called 'The form of the Good.'
Many philosophers think this heavily impacted the story of Jesus, and much of the writings in the Bible, but that's detour for another discussion.
This idea was the initial form of deism - belief in a single God who doesn't interfere with the Universe or human affairs.
Worshiping it would be pointless because it does not care. No money can be made from this ideology because there is no doctrine or belief system to follow.
You can say that individual religions might have been started by conspiracies.
I think Christianity has a strong case in that regard, but that is not to say that all Christian preachers today are 'in on it.'
Yes, there have been conspiracy theories within Churches recently, the Catholic church springs to mind for some reason, but you also have religions in which the core tenants involve giving up worldly goods.
Jainism, Buddhism, Taoism. All of these religions encourage an individual journey which usually involves a lot of giving away your possessions to the poor, and not a lot of flying around the world in expensive jets paid for by Church donations.
To conclude, I don't think that God is a conspiracy theory.
I think that, on a case by case basis, through every different religion - and every different denomination within those religions - conspiracies can be found, but the concept of a God was simply an idea which evolved alongside our species. Plato had no ill intentions in his writings, and was actually very controversial in his dialogues - not a great thing to be at the time considering his teacher, Socrates, was executed for 'corrupting the youth' - so it wouldn't make sense for him to invent the concept of a singular God for monetary gain.