The Church of South India (commonly known as CSI) is the successor of the Church of England in India. It came into being by a union of Anglican and Protestant churches in South India. With a membership of over 5 million, it is India's second largest Christian church after the Catholic Church in India. CSI is one of four united churches in the Anglican Communion.

The inspiration for the Church of South India was born from ecumenism and inspired by the words of Jesus Christ as recorded in the Gospel of John, 17.21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.

That they all may be one is also the motto of the Church of South India.