The Coonan Cross Oath was a crucial event for the Eastern Church community in India, particularly impacting the followers of the Saint Thomas Christians in Kerala. This movement arose as a response to growing discontent with the Portuguese colonial influence and their attempts to enforce Latin Christianity on the local Christian populace. The event culminated in Mattancherry on January 3, 1653, when a large number of the local Christians, led by the community leaders, took an oath at the Coonan Cross. Under the leadership of figures like Archdeacon Thomas and others, they gathered at the Coonan Cross to declare their commitment to upholding their traditional beliefs and rejecting foreign domination in spiritual matters.

The oath itself was a powerful statement against the heretical imposition of the Portuguese style of governance and church practices, asserting a desire to return to the roots of their faith, as practiced by the early Christian community in India. This act was not purely religious but also a social assertion of identity and governance, as they sought to establish their autonomy from Portuguese control, which had increasingly alienated them from their cultural and religious heritage.

The aftermath of the Coonan Cross Oath led to significant political and religious implications. Led by the Archdeacon, who was one of the key figures in the movement, the faction that took the oath split from the Catholic Church aligned with the Portuguese and instead sought to reaffirm their connections with the ancient Eastern Christian traditions. This separation initiated a new phase in the history of the Saint Thomas Christians, leading to the formation of the independent Syrian Christian Church and eventually contributing to the diversity of Christian denominations present in India today.