Get/be on your soapbox
Definition 1 of 1
Pro Tip 1/3Public Speaking
Used when someone is sharing their opinions with many people, like giving a speech. 
At the rally, she got on her soapbox to address climate change.
Pro Tip 2/3Strong Feelings
Indicates strong emotions or passion about the discussed topic. 
He was on his soapbox, arguing vehemently for animal rights.
Pro Tip 3/3Colloquial Use
It's informal and best suited for casual conversation, not formal writing. 
My friend often gets on his soapbox about junk food.