credentials vs testimonial

credentials

noun
  • Documentary evidence of someone's right to credit or authority, especially such a document given to an ambassador by a country. 

  • login data 

testimonial

noun
  • A statement, especially one given under oath; testimony 

  • A match played in tribute to a particular player (who sometimes receives a proportion of the gate money). 

  • A tribute given in appreciation of someone's service etc. 

  • A written recommendation of someone's worth or character 

adj
  • Serving as testimony. 

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