Handel wrote this letter in response to a draft sent to him by Charles Jennens, a gentlemen from a noble family from the Midlands, who supervised the assemblage of the text of the Messiah. Handel would have been living in the Brook Street house at this time, and, as the coversheet shows, Jennens was living in Leicestershire.
By the time this letter was written, the Messiah had already been performed. But Jennens wasn't happy with all of Handel's music for the Messiah; he didn't feel it always did justice to the subject - which was a hell of a cheek, you might think. After all, it is for Handel's music that the Messiah is remembered, not for Jennens's words.
In the letter, Handel is acknowledging the piece which was to become the biblical drama Belshazzar. He clearly liked it, and in a further letter, owned by a collector in New York, he discusses further points about it. Handel goes on to say thank you, and that it has given him lots of ideas, but he then asks what is wrong with his music for the Messiah: "Be pleased to point out these passages in the Messiah which you think require altering." Jennens probably did tell him, but that's not in writing.
His objections may well have been related to his religio-political position. Jennens was a Nonjuror - a Protestant who refused to swear an oath of allegiance to the Hanoverians - and, as such, felt that the divine right of monarchy belonged not to the Hanoverians, but to the Catholic Stuarts. So they were in a very uneasy predicament: they believed passionately in Protestantism on one hand, but were very uneasy about its upholders on the other. This probably influenced the way he wrote certain things, but Handel was a musician above all else and was probably blissfully unaware of this. Scott Hughes