Significance of '-fu' suffix on Swahili nouns?

What is the difference between the Swahili words 'mwamini' and 'mwaminifu'? I've read that the latter carries a connotation of 'true' or 'authentic', such as 'true believer' (vs. just 'believer') derived from the verb 'kuamini'. But I can't find anything about the '-fu' suffix that causes this connotation. Anybody know?