Messiah (not The Messiah) was written by Handel in 1741, reputedly in 21 days. The libretto was compiled by Jennens, being a collation of quotations from the King James Version of the Bible. Neither Jennens or Handel regarded Messiah as a religious work. Messiah was composed specifially for a series of oratorio concerts that Handel was putting on in Spring 1742, in Dublin.
Handel's original plan had been to ask some of the choristers from Dublin Cathedrel to sing. The Dean of the Cathedral (Jonathan Swift - author of Gulliver's travels) refused to let any of his choristers take part in "Mr Handel's little entertainment" on the grounds that there was insufficient religious content. Undeterred, Handel recruited choristers from the Cathedral in Chester, where he was waiting to take the boat to Ireland. He asked the choirmaster for choristers who could "sing by sight". Handel himself relates the story that while waiting in Chester for the storms to abate, he began rehearsals for Messiah in the Bear, and was much put out that his bass was unable to sightread "Why do the Nations". He enjoyed the response of the bass - that he could sing at sight , but "not first sight".
Messiah is unusual as an oratorio, it does not relate a specific story. Nor do the soloists sing particular roles. Jennens designed the work as a contemplation. Part I tells of the prophecies of the coming of the Messiah, and the Nativity. Part II tells of the Passion, the Resurrection and the Ascension into Heaven. Part III considers the Word that is left behind. The initial performance of the Messiah was not a great success, but gradually the work grew in popularity. Whenever it was performed, Handel would rearrange and rescore to take advantage of whatever resources were available, so it is not possible to arrive at a definitive score for Messiah. The Brickhills Messiah performance in 2007 uses the 1751 score that Handel compiled for a charity performance of the Messiah in aid of his Foundling Hospital.