lack of faith vs niggle

lack of faith

niggle

verb
  • To use, spend, or do in a petty or trifling manner. 

  • To dwell too much on minor points or on trifling details. 

  • To fidget, fiddle, be restless. 

noun
  • A minor complaint or problem. 

How often have the words lack of faith and niggle occurred in a corpus of books? (source: Google Ngram Viewer )