We're hiding the Holy Grail to protect it from the evils of this world
I have found the Holy Grail
The Holy Grail is the blood of Jesus Christ. The Knights Templar's hid it to keep the evils of this world from profiting or using it for evil. They hid it so people wouldn't use it for Paganism. The Pagans wanted to destroy the Holy Grail because It went against there religion.
In the movie Indiana Jones. He actually finds the Holy Grail.
I've found El Dorado
This saying means you have found the best of something. Often people will say they have found the Holy Grail of all foods. But in the Religious category it refers to Jesus Christ dying on the cross and his blood being kept in a golden vase hidden from society.
El Dorado is the city of gold. The city is supposedly where all the trade was happening in central Africa when the Great Gold and Salt Caravan would come through. They would go to El Dorado and store the kings gold there.
This explorer has dedicated his life to trying to find the great lost city of gold or El Dorado. He has spent all of his funding and grants to get to this point.
El Dorado is literally the city of gold. When someone says the have found El Dorado is typically means they have found something extremely rich or has a lot of value or meaning to the,. Also, it can mean they found something amazing to them.