- http://www.newadvent.org/cathen/08010c.htm
- http://www.newadvent.org/cathen/12043b.htm
- http://www.newadvent.org/cathen/11054a.htm
- http://www.newadvent.org/cathen/04737b.htm
- http://www.newadvent.org/cathen/11549a.htm
- http://en.wikipedia.org/wiki/Pope_Nicholas_I
- http://inquisitionupdate.wordpress.com/2012/11/27/the-papacy-and-the-civil-power-pages-364-370/
- http://www.catholic.org/saints/saint.php?saint_id=1020>