The text to "Adam Lay Ybounden" dates all the way back to the 15th century, but, according to thehypertexts.com, the original tune of the carol has not survived. However, as Wikipedia notes, in more recent years the text of the carol has been set to music by a number of different composers, such as the British composers Peter Warlock (1894-1930) and John Ireland (1879-1961). It was also incorporated into Benjamin Britten's (1913-1976) choral work A Ceremony of Carols, under the title "Deo Gracias" (note: I try to avoid using Wikipedia as a source given its questionable reliability in the case of controversial topics, but given that the topic at hand is relatively noncontroversial, I feel comfortable citing it here). Here are the words of the carol in the original Middle English:
Adam lay ybouden,
Bounden in a bound;
Four thousand winter thought he not too long.
And all was for an apple,
An apple that he took,
As clerkes finden written in their book.
Ne had the apple taken been,
The apple taken been,
Ne had never Our Lady,
A-been heaven's queen.
Blessed be the time
That apple taken was!
Therefore we may singen
Deo gracias! [Latin: thanks be to God!]
Here is a version by Michael R. Burch in more modern English:
Adam lay bound, bound in a bond;
Four thousand winters, he thought, were not too long.
And all was for an apple, an apple that he took,
As clerics now find written in their book.
But had the apple not been taken, or had it never been,
We'd never would have had our Lady, heaven's queen.
So blessed be the time that apple was taken thus:
Therefore we sing, "God is gracious!"
The text of the carol is based on the medieval Catholic belief that Adam and Eve and the other Old Testament patriarchs remained in the limbus patrem--in an intermediate state, neither in Heaven or Hell--after their deaths until the Crucifixion of Christ ("four thousand winter"), after which they entered into Heaven. The "book" of the "clerkes" (i.e., clerics or priests) probably refers to the Bible. The "apple" reflects the widespread belief that the forbidden fruit in the Garden of Eden that Adam and Eve took in defiance of God's command was an apple. The message of the carol is that the Fall of Man (Adam and Eve's disobedience to God) was actually a good thing in that it made it possible for Mary ("Our Lady"), the mother of Jesus, to become "heaven's queen"--reflecting the devotion of medieval Catholics to Mary.
Interestingly, though, the text of this carol not only reflects the beliefs of the medieval Catholic Church, but also--to my way of thinking at least--demonstrates why the Protestant Reformation of the sixteen century was necessary. For one thing, it exhibits--at least to a Protestant like me--an excessive devotion to Mary. Mary is certainly to be highly honored for her willingness to become the human mother of Christ. However, to exalt her to the point of giving her the title "Heaven's Queen"--a title not sanctioned by Scripture--is excessive and verges on idolatry. Moreover, the title "Heaven's Queen" or "Queen of Heaven" in and of itself is problematic. According to the Encyclopedia Britannica, "Astarte/Ashtoreth is the Queen of Heaven to whom the Canaanites [of Old Testament times] burned offerings and poured libations (Jeremiah 44)." In other words, medieval Catholics were applying a title originally used for a pagan goddess to the mother of Jesus! This seems quite inappropriate.
Perhaps even more concerning is the fact that the text of the carol implies that the Fall of Man was not really a tragic event but rather one for which Christians ought to thank God. This is in complete contrast with the way in which the New Testament consistently portrays the Fall of Man--as the very reason for which Christ had to die on the cross. This idea that the Fall of Man was a positive development is actually closer to the heretical belief of the Mormons that the Fall was "a necessary part of the overall plan of redemption," (see here for an official Mormon statement) than orthodox Christian belief.
Finally, note the reference to "their book"--that is, the book of the "clerkes." From the perspective of the speaker in the text of the carol, the Bible belonged to the clergy. It did not belong to the ordinary Christian. This reflects the widespread idea in medieval Catholicism that ordinary believers should not read the Scriptures themselves, but should rely on a priest to tell them what it said. It also is reflective of the fact that most lay believers were illiterate; only the clergy had the ability to read. An added problem was the the Bible was essentially only available in Latin, a language only the educated would know. This was a situation that the Reformers worked hard to reverse. Strongly believing that ordinary Christians should read the Bible for themselves, they created translations of the Scriptures into German, English, and other languages. They encouraged literacy. They would have strongly rejected the notion that the Bible belonged only to the "clerkes."
In short, "Adam Lay Ybounden" shows quite clearly why the Protestant Reformation was necessary. The text of the carol demonstrates how far medieval Catholicism had drifted from the Biblical foundations of Christianity.
To conclude, whatever we may think of "Adam Lay Ybounden" in its modern musical incarnations, we should be wary of its theology. Perhaps some readers will think that I am being hyperbolic about this, but I would say that at the very least it is important for us to be discerning about what we read, watch, or listen to--however attractive it might be otherwise.
Image of Hans Holbein's "Adam and Eve" from Wikimedia Commons