In works such as Water Buffalo Theology and Three Mile an Hour God, he defended a theology that he considered to be accessible to the peasantry in developing nations, rather than an overly academic systematic theology. In total, Koyama wrote thirteen books. One of his most well-known books, "Water Buffalo Theology", was described as "ecological theology, liberation theology and contribution to Christian-Buddhist dialogue". Water Buffalo Theology is probably Koyama's best-known work. The book was partly inspired by Koyama's work as a missionary in Northern Thailand. His works of Mount Sinai and Mount Fuji and Water Buffalo Theology are, in part, an examination of Christian theology within the context of Thai Buddhist society, growing out of Koyama's missionary experience in Thailand. Koyama was an editor of the South East Asia Journal of Theology, for which he himself wrote a considerable number of articles. Koyama published at least thirteen books, including "On Christian Life" and over one hundred scholarly articles. Koyama's work has been described as helping to bridge the boundaries between East and West, between Christianity and Buddhist thought, between the rich and the poor. It has been pointed out that he has no overarching system in this theology, which shows commitment to serving a "broken Christ trying to heal a broken world". He was named as an important figure for the development of a world Christianity. In No Handle on the Cross: An Asian Mediation on the Crucified Mind, Koyama explained how the cross can be considered the symbol of Christian suffering, and began the book with a chapter entitled "The Cross and the Lunchbox". Koyama explained, in the preface to this book, which he wrote in Tokyo at Christmas 1975, how he did Christian missionary work in Thailand from 1960 to 1968, and how his experiences in Thailand rekindled interest in Asian religion in him. He also explained in the preface how the book grew out of the Earl Lectures which he delivered at the Pacific School of Religion in California. Koyama also spoke at a conference in Edinburgh in 1985, in which he described God as "a hot God", suggesting a certain dynamic quality to the attributes of God. In this book, he reflected on his interest in the relationship between history and theology, and pointed out the imperial nature of Indo-European languages among those speakers of languages such as Sinhalese or Japanese. He also told, in this book, of an incident which happened to him in Singapore, where he met a Buddhist monk from Sri Lanka. He told the monk about crusades for Jesus Christ and the monk looked puzzled. "Why do you need to crusade?" asked the monk. The monk indicated that if one felt a need to crusade, then one was not a true disciple. Instead, the monk championed, true to his Buddhist beliefs, the virtue of self-denial. His last work was Theology and Violence: Towards A Theology of Nonviolent Love, published in Japanese in 2009 by Kyobunkwan, a publishing firm in Tokyo.
