Rison -- who played for Gruden and the Raiders in the 2000 NFL season -- said despite the vile, racist, homophobic and misogynistic words Jon wrote in emails from 2011 to 2018 ... he still believes the guy is a good man.

"We all say some things behind closed doors that we regret or we wouldn't say in public," said Rison, who's remained close to Gruden even after the '00 season ended. "We've all made mistakes."

"I believe who he said and what he said and who he intended it for, it was intended for that person and nobody else."

Rison did admit Gruden did mean what he said in his messages to a Washington Football Team exec and others ... and offered to apologize on the coach's behalf.

But, Rison added in all the years he's been around the 58-year-old, he's never seen anything that would suggest Gruden is a bad person.

Rison also told us he believes if this is the end of Gruden's time in the NFL, the former Raiders head man should hold his head high.

"You've already given this game some of the best tutelage and best leadership that you could possibly give," Rison said in a message to Gruden. "So if you don't ever touch the game again, don't feel any type of way."