In the first of our Worship 101 series on Holy Trinity Sunday, we tackle a big fancy word: Invocation. Why do we call upon the names of God in our worship? Because names have power. Featuring The Doctor (and Martha Jones).

Image: Jacques Callot, The Holy Trinity in the Tree of Life, Adored by Franciscans, French, 1592 - 1635, in or after 1621, etching, Andrew W. Mellon Fund (from the National Gallery)