The Coonan Cross Oath, also known as the Great Oath of Bent Cross, the Leaning Cross Oath or the Oath of the Slanting Cross, taken on 3 January 1653 in Mattancherry, was a public avowal by members of the Saint Thomas Christians of the Malabar region in India, that they would not submit to the Jesuits and Latin Catholic hierarchy, nor accept Portuguese dominance in ecclesiastical and secular life.

The Saint Thomas Christians, also called Syrian Christians of India, Marthoma Suriyani Nasrani, Malankara Nasrani, or Nasrani Mappila, are an ethno-religious community of Indian Christians in the state of Kerala, who, for the most part, employ the Eastern and Western liturgical rites of Syriac Christianity. They trace their origins to the evangelistic activity of Thomas the Apostle in the 1st century. The Saint Thomas Christians had been historically a part of the hierarchy of the Church of the East but are now divided into several different Eastern Catholic, Oriental Orthodox, Protestant, and independent bodies, each with their own liturgies and traditions. They are Malayalis and their mother tongue is Malayalam, which is a Dravidian language. Nasrani or Nazarene is a Syriac term for Christians, who were among the first converts to Christianity in the Near East.