Religion is without a doubt man-made. There is no book that could not have been made by a human or has any hint that it was not made, or dictated, by a human. There is no organization (the catholic church, the jewish synygogues, the buddhist temples, etc) that doesn't have humans as 100% of it's membership and that is made by humans.
What cannot be proven to be man made are the gods that religion talks about.
Think of it this way: astrology, and to much lesser extent astronomy, is man-made but the stars are not. What we say about the stars is 100% man-made... what the stars are on the other hand are wholly independent of humans.
Humans have deceived, scammed, and conned each other throughout history in all sorts of ways. If humans can scam one another out of money and power (just for example), don't you think they could easily scam each other for control through the invention of pretend super powers?
What other creature on this planet has the imagination, creativity, and need for attention than a human? Our history is filled with imaginative story-tellers as well as greedy power hungry people who lie, cheat, manipulate, and take advantage when the opportunity presents itself. Leaders past and present have taken advantage by creating fear with misinformation and used religion to gain control.
If a deity needs recognition and adulation then it would present itself to all humans, it is inefficient otherwise.
The faithfools don't accept science, they don't accept reason, they don't accept modern times, they do not accept the reality...... how would they accept the proof the God is man-made?
A woman in the U.S. just saw Jesus standing in the burning spire of Notre Dame on TV. This is in 2019? The faithfools see Jesus in bread toast. They see Mother Mary crying blood. All this defies logic and reason. You cannot convert the stupid. Stupid is by choice. Once they have made it, they cannot be changed. They must die out.
Whether or not there are any gods, and I believe there are not, religion itself is a social construct. If there were gods, they would continue to exist even if no one worshiped them, just as people worship even though there are no gods. Religions have more or less evolved over the millennia to fit the needs of their societies, so without knowing exactly when and where the first religion started, it would be impossible to prove the origin of religion, just as it is impossible to "prove" the nonexistence of gods or other unlikely beings.