"O youth deprived of understanding, who has bewitched you into believing, that the Supreme and Eternal is eaten by you and held in your intestines?" - Baruch Spinoza, letter to Albert Burgh (Dec. 1675), as quoted in Betraying Spinoza
Burgh had been a student of Spinoza's, but during a trip to Italy he converted to Roman Catholicism and sent Spinoza a letter in September vehemently denouncing Spinoza's philosophy. Spinoza died two months after sending his response.*
Debunking Christianity has a full text of the exchange.
*Some sites list Spinoza's death in 1677, nearly two years after the letter. I'm using the date given by Goldstein in Betraying Spinoza.