These people are just living their ordinary lives when a miracle happens: one of the most beautiful hymns in the world, Handel's Hallelujah, breakes out of nowhere and fills all the air around them. Sure an unforgettable experience. (notice how many people also join in the singing together with the professionals and some even shed a tear or two)

This song is the chorus of Handel's most famous work, the Messiah. "Messiah" is an English-language oratorio* composed by George Frideric Handel. It is one of the most popular works in the Western choral music and has become a entrepiece of the Christmas season. The song was composed in London during the summer of 1741 and premiered in Dublin, Ireland on 13 April 1742. Handel was born German but then lived in London and naturalized British.

*An oratorio is a large musical composition including an orchestra, a choir, and soloists. Like an opera, an oratorio includes the use of a choir, soloists, an ensemble, various distinguishable characters, and arias. However, opera is musical theatre, while oratorio is strictly a concert piece.

At noon, on November 13, 2010, these unsuspecting shoppers got a surprise while enjoying their lunch.

Hallelujah! Hallelujah!
Hallelujah! Hallelujah! Hallelujah!

Hallelujah! Hallelujah!
Hallelujah! Hallelujah! Hallelujah!

for the Lord God Omnipotent reigneth.
Hallelujah! Hallelujah!  Hallelujah! Hallelujah!

for the Lord God Omnipotent reigneth.
Hallelujah! Hallelujah!  Hallelujah! Hallelujah!

for the Lord God Omnipotent reigneth.
for the Lord God Omnipotent reigneth.
Hallelujah! Hallelujah!
for the Lord God Omnipotent reigneth.
Hallelujah!

The kingdom of this world is become
the Kingdom of our Lord and of His Christ
and of His Christ
and He shall reign for ever and ever
and He shall reign for ever and ever
and He shall reign for ever and ever
and He shall reign for ever and ever

King of kings
for ever and ever. Hallelujah! Hallelujah! 
and Lord of lords
for ever and ever. Hallelujah! Hallelujah! 
King of kings
for ever and ever. Hallelujah! Hallelujah! 
and Lord of lords
for ever and ever. Hallelujah! Hallelujah! 
King of kings
for ever and ever. Hallelujah! Hallelujah! 
And Lord of lords
King of kings, and Lord of lords,
and He shall reign for ever and ever
and He shall reign for ever and ever

King of kings for ever and ever
and Lord of lords. Hallelujah! Hallelujah!
and He shall reign for ever, for ever and ever,

King of kings,
and Lord of lords,
King of kings,
and Lord of lords,
and He shall
and He shall reign for ever and ever,

For ever and ever // King of kings
For ever and ever // and Lord of lords

Hallelujah! Hallelujah!.
Hallelujah! Hallelujah!.
Hallelujah!

UNSUSPECTING= If you are unsuspecting, you have no idea that something is happening or going to happen (you don't suspect anything).

HALLELUJAH= /(h)æləlu:jə/ (Hebrew word from the Bible, also spelled Halleluyah, Halleluiah or Alleluia) A joyful word of praise to God, something like "glory be to God" or, in modern language, "God rules!", "He's the best!". The strange J that we pronounce /j/ (like an Y) here is an old spelling that has survived because it was used in the standard Bible for the English (King James Version), but you can also use the I (or even Y), which fits better with its pronunciation. Spellings without H are also possible because you can also pronounce this word without /h/.
We can also use this word to express a great joy for something good that happened, specially after a long wait (e.g. "halleluiah! you finally passed your driving test!")

LORD= God is often called "the Lord" because he has power over everybody the same as in all times a lord (or king) had power over all the people living in his lands. You can also say "the Lord God", though it is a bit redundant.

OMNIPOTENT= All-powerful, having unlimited power (from Latin omnia=all things + potens= powerful)

REIGNETH= (old fashioned or literary) /reɪnɪθ/ Reigns, rules. A king reigns (exercises power) over his people.
In Old English, the third person singular of the present tense added –ETH /ɪθ/. But in the north of England, due to Danish influence (they were conquered by the Vikings), they started to add –ES, and that is the form that finally became standard a few centuries ago as -S, so today we say "he walks" and not "he walketh", though the –ETH form is still used in poetry and the standard KJV* Bible translation.  (*King James Version).

OUR LORD= God

HIS CHRIST= Jesus.
The word "christ" /kraɪst/ is the Greek translation of the Hebrew word "messiah" /məsaɪə/, which means "the anointed one" (the one chosen by God to lead His people). In ancient times God promised to send a messiah to save His people. Christians believe that Jesus was the messiah promised by God, so they called Jesus "the Messiah". Since most of the New Testament was written in Greek, He was called "the Christ", and very soon people mixed both names and started to call Him "Jesus the Christ" or "Jesus Christ", or simply "Christ" used as a proper name.
Latin people used the spelling CH to transcribe the Greek sound /x/ (which sounds like a strong "h", as in the Scottish "loch" or the Spanish "Juan"). The Greek word Xristos was pronounced /xristos/ and spelled Christus in Latin. Of course, most people pronounced it with a Latin accent: /kristus/. In English, most of those ancient Greek words originally pronounced with /x/ and transcribed with CH still keep the CH spelling but have the Latin pronunciation /k/, for example in: Christ, mechanic, technician, chaos, anarchist, monarchy, anachronism, chromatic, archaeology, archetype, archaic, architect, psychology, melancholy, technology, schizophrenia, character, chemical, chorus, chronometer. All these words, and many more, are pronounced with /k/ in English.

SHALL= (old fashioned or emphatic) Will.

FOR EVER AND EVER= An emphatic form of "forever", always, for all eternity.
So the sentence "He shall reign for ever and ever" means "He will be king eternally".

KING OF KINGS= The most powerful king and lord of all the other kings; or the one and only king.
Religious expressions like "king of kings" and "lord of lords" are actually a direct translation of a Hebrew grammar structure used to express superlative, so "king of kings" really means "the most important king".

LORD OF LORDS= The most important ruler (see above)

Messiah presents an interpretation of the Christian view of the Messiah, or "the anointed one" as Jesus the Christ. Divided into three parts, the libretto covers the prophecies concerning the Christ, the birth, miracles, crucifixion, resurrection and ascension of Jesus, and finally the End Times with the Christ's final victory over death and sin. [This future victory of Christ depicted in the Book of Revelation is what the chorus "Hallelujah" sings about].

Although the work was conceived for secular theatre and first performed during Lent, it has become common practice since Handel's death to perform Messiah during Advent, the preparatory period of the Christmas season, rather than in Lent or at Easter. Messiah is often performed in churches as well as in concert halls.

Although Messiah deals with the New Testament story of Christ's life, a majority of the texts used to tell the story were selected from the Old Testament prophetic books of Isaiah, Haggai, Malachi, and others.

HALLELUJAH

The most famous movement [in this oratorio] is the "Hallelujah" chorus [the fragment shown on this video]. The text is drawn from three passages in the New Testament book of Revelation:

And I heard as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying, Alleluia: for the Lord God omnipotent reigneth. (Revelation 19:6)

And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever. (Revelation 11:15)

And he hath on his vesture and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS. (Revelation 19:16)

In many parts of the world, it is the accepted practice for the audience to stand for this section of the performance. The tradition is said to have originated with the first London performance of Messiah, which was attended by King George II. As the first notes of the triumphant Hallelujah Chorus rang out, the king rose to his feet and remained standing until the end of the chorus. Royal protocol has always dictated that when the monarch stands, everyone in their presence is also required to stand. Thus, the entire audience and orchestra stood when the king stood during the performance, initiating a tradition that has lasted more than two centuries. It is lost to history the exact reason why the King stood at that point, but the most popular explanations include:

•    He was so moved by the performance that he rose to his feet.
•    As was and is the custom, one stands in the presence of royalty as a sign of respect. The Hallelujah chorus clearly places Christ as the King of Kings. In standing, King George II accepts that he too is subject to the Lord of Lords.

There is another story told about this chorus that Handel's assistant walked in to Handel's room after shouting to him for several minutes with no response. The assistant reportedly found Handel in tears, and when asked what was wrong, Handel held up the score to this movement and said, "I thought I saw the face of God".

(extracted from Wikipedia)