Believing strongly in a religion and obeying all its rules or principles. (Cambridge Dictionary)
Committed or devoted to religion or to religious duties or exercises. (Merriam – Webster)
A devout person has deep religious beliefs. (Collins Dictionary)
If you describe someone as a devout supporter or a devout opponent of something, you mean that they support it enthusiastically or oppose it strongly. (Collins Dictionary)
Having or showing deep religious feeling or commitment. (Oxford Dictionaries)
Totally committed to a cause or belief. (Oxford Dictionaries)
Devout. Your Dictionary.