
The painting is from the massive Ghent altarpiece, "The Adoration of the Lamb" by Hubert and Jan van Eyck at St. Bavo Cathedral, Ghent, Belgium.
Wiki shows the whole altarpiece, except for missing parts, and it is worth a look.
The well-known carol, “O come, O come, Emmanuel,” provides just such a passageway linking the old and the new. The carol’s familiar names for Christ are based on the Advent Antiphons—the “Great O’s”—which date back possibly to the sixth century. These antiphons—short devotional texts chanted before and after a psalm or canticle—were sung before and after the Magnificat, the Song of Mary, at Vespers from December 16 through December 23. Each of the antiphons greets the Messiah and ends with a petition of hope. The simple refrain of the carol, “Rejoice! Rejoice! Emmanuel shall come to thee, O Israel!” sets the tone for this Advent time of waiting and expectation.From "Hasten the Kingdom: Praying the O Antiphons of Advent" by Mary Winifred, C.A. (Liturgical Press, 1996).
Over the next several days, beginning today, I plan to post the "O Antiphon" of the day.
Reposted from last year with slight editing.