Dyed in the wool
Definition 1 of 2
Pro Tip 1/3Identity Indicator
Use when describing someone's unwavering support or commitment that's part of their identity. 
Aunt May is a dyed-in-the-wool supporter of animal rights.
Pro Tip 2/3Cultural Loyalty
Often associated with deep cultural or political beliefs that one is very loyal to. 
He's a dyed-in-the-wool fan of traditional jazz music.
Pro Tip 3/3Metaphorical Color
The phrase originates from the permanence of wool's color after dying, symbolizing steadfastness. 
My teacher's a dyed-in-the-wool grammarian, insisting on perfect syntax.