Aaron Hernandez wrote a letter from jail, which TMZ somehow obtained, declaring his innocence, defiantly stating his innocence, and, naturally, shouting out haters. 

Here's a small sample. 

Aaron Hernandez wrote a pen pal a letter from inside his jail cell, declaring his innocence, saying his murder charges are God's plan, and vowing to return to the NFL ... and TMZ has a copy.

We have verified with law enforcement the letter is the real deal. You can read it yourself ... among the highlights:

-- "The world just makes things out of false accusations ... and it will all die down, especially when they say NOT GUILTY"

-- "All the people who turned on me will feel like crap." -- "God put me in this situation for a reason."

-- "Can't wait to sign [autographs] again when I'm playing again an [sic] prove all the haters WRONG."