TMZSports.com: 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."
As for the fan who received the letter ... he wrote Hernandez first. The fan tells us ... he sent a letter because he was in the same jail 15 years ago, and wanted to reach out to Hernandez to give him some encouragement.
FYI, Hernandez -- who's been accused of murdering semi-pro football player Odin Lloyd -- has been denied bail and will remain behind bars until his trial.
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