Veni Redemptor Gentium is one of the earliest known Western Church hymns dating to the 4th century A.D. The hymn is considered to be in the genre of Ambrosian Chant, predating Gregorian Chant, as it is believed to be written by St. Ambrose of Milan.
The slideshow does not have the presenter to fill in gaps, so a little St. Ambrose backstory:
In 303 A.D., The Roman court was moved from Rome to Milan. The Emperor Valentinian I had called St. Ambrose to serve as the governor of Milan, which due to the location of the court gave St. Ambrose an immense amount of power. The Arian bishop had died in Milan causing a conflict in the streets between Arians and Catholics. St. Ambrose attempting to calm the riots went to the Cathedral to prevent a city riot. The passion of St. Ambrose moved both sides as both began chanting “Ambrose for bishop!”¹
St. Ambrose, after being ordained Bishop of Milan on December 7th, 374 A.D. began teaching the faithful in his church a new style of singing called antiphonal. While teaching melodies created by himself, St. Ambrose inspired by both Psalms and his own creativity composed hymns that had not been common in the Western Church. St. Ambrose’s chant “is the earliest surviving written music of Western Civilization.”²
¹Marcellino D’Ambrosio, When the Church Was Young: Voices of the Early Fathers (Cincinnati: Franciscan Media. 2014), 211.