In 2012 the NFL had an issue with Tim Tebow kneeling for each game to pray, they also had an issue with Tebow wearing John 3:16 as part of his blackout to avoid glare and made him take it off.
In 2013 the NFL fined Brandon Marshall for wearing green cleats to raise awareness for people with mental health disorders.
In 2014 Robert Griffin III (RG3) entered a post-game press conference wearing a shirt that said “Know Jesus Know Peace” but was forced to turn it inside out by an NFL uniform inspector before speaking at the podium.
In 2015 DeAngelo Williams was fined for wearing “Find the Cure” eye black for breast cancer awareness.
In 2015 William Gay was fined for wearing purple cleats to raise awareness for domestic violence. (not that the NFL has a domestic violence problem).
In 2016 the NFL prevented the Dallas Cowboys from wearing a decal on their helmet in honor of five Dallas Police officers killed in the line of duty.
In 2016 the NFL threatened to fine players who wanted to wear cleats to commemorate the 15th anniversary of 9/11.
h/t GorT, Sr.