The "Agnus Dei" was an important part of Catholic Mass and many chorale compositions. The words translated into English are "Lamb of God."

A contemporary "Agnus Dei" was written by Michael W. Smith, and is well known today.  Along with John 1:29, it draws upon the book of Revelation in the Bible.

John 1:29  "The next day, John saw Jesus coming unto him, and said, "Behold the Lamb of God, which taketh away the sin of the world."

Revelation 11:17  "Saying, We give you thanks, O Lord God Almighty, which art, and was, and are to come:  because You have taken Your great power and reigned."

There are four lines that are repeated:

Alleluia

For the Lord God Almighty reigns

Holy

Worthy is the Lamb.