The line "He who works for sweetness and light united, works to make reason and the will of God prevail" is from Matthew Arnold's book "Culture and Anarchy." This book, published in 1869, is a collection of essays in which Arnold critiques the social, cultural, and political issues of his time.
In "Culture and Anarchy," Arnold advocates for the pursuit of culture, which he defines as the pursuit of "sweetness and light." By "sweetness," he means beauty and aesthetics, and by "light," he refers to knowledge and truth. Arnold argues that the combination of these elements can lead to a more enlightened and harmonious society. The phrase "sweetness and light" became well-known as a result of his writings.