When Handel was commissioned to write Messiah, his previously popular Italian opera had become rather passe, even parodied. English language oratorii were the new thing, he had had some success with them already, but perhaps he was feeling down on his luck [as suggested in the Alpha presentation I watched on Saturday].
He wrote the 259 page work in 24 days, there are remarkably few errors or corrections for such a long piece, although Handel (and his contemporaries) had written at such a speed previously, and his next piece was also completed in a month. As usual, he rehashed earlier works.
The piece was originally conceived for a modest orchestra, it was later that performances became more elaborate and lavish.