characterized by a firm and humorless belief in the validity of your opinions
"both sides were deeply in earnest, even passionate"
"an entirely sincere and cruel tyrant"
"a film with a solemn social message"
dignified and somber in manner or character and committed to keeping promises
"a grave God-fearing man"
"a quiet sedate nature"
"as sober as a judge"
"a solemn promise"
"the judge was solemn as he pronounced sentence"