The reference is to the Ter Sanctus or Triumphal Hymn, which is found in every Liturgy. The previous writer referred to is thought by some to be S. Athanasius, but by others S. Dionysius the Areopagite, who has some words on this point in his treatise De Cœlest. Hier., c. 7. But the most competent scholars deny the authenticity of the works attributed to S. Dionysius, and place them from one hundred to one hundred and fifty years later than S. Gregory’s time.