More Gregorian chants, following the selection in Rutter's Eleven Gregorian Chants.
Vexilla Regis
Text (and possible melody) by Venantius Fortunatus (c.530-c.600/609), who composed this hymn in 569 to celebrate a procession bringing a fragment of the True Cross to Poitiers.  Unsurprisingly, it is concerned with the Cross and Christ's Passion; in the Church calendar, it is sung at Vespers in Passiontide (Holy Week).  A selection of Venantius' poems can be found on the Latin Library, including the text below. More Venantius poems, with facing translation, are printed in Helen Waddell's Medieval Latin Lyrics (4th ed., 1933), 58-67.
Vexilla regis prodeunt 
Vexilla regis prodeunt
Fulget crucis mysterium
Quo carne carnis 
conditor
Suspensus est patibulo. 
Quo vulneratus insuper
Mucrone diro lanceae
Ut nos lavaret 
crimine
Manavit unda et sanguine. 
Impleta sunt quae concinit
David fideli carmine
Dicens In 
nationibus
Regnavit a ligno Deus. 
Arbor decora et fulgida
Ornata Regis purpura
Electa digno 
stipite
Tam sancta membra tangere. 
Beata, cujus brachiis
Saecli pependit pretium
Statera facta 
corporis
Praedamque tulit tartari. 
O Crux ave, spes unica
In hac triumphi gloria
Auge piis 
justitiam
Reisque dona veniam. 
Te summa Deus Trinitas
Collaudet omnis spiritus:
Quos per crucis 
mysterium
Salvas, rege per saecula. Amen. 
For text with Blount's 1717 translation, see here.
Text and literal translation here
Performance by Schola Gregoriana Mediolanensis
Text, video and Gregorian notation are given by the excellent Brasil-based gregoriano.org site
Two Hymns by Thomas Aquinas
Pange Lingua
Text written in 1263 by St Thomas Aquinas, to an earlier melody. Hymn sung at the Feast of Corpus Christi.
Adoro te Devote
Eucharistic Hymn.