
The Catholic Church, also known as the Roman Catholic Church. This is the largest Christian church, with approximately 1.3 billion baptised Catholics worldwide, as of 2016. As one of the oldest religious institutions in the world, it has played a prominent role in the history and development of Western civilisation. The church is headed by the Bishop of Rome, known as the Pope. Its central administration, the Holy See, is in the Vatican City, an enclave within Rome, Italy. Catholic theology is based on the Nicene Creed.

The Christian doctrine of the Trinity (Latin: trinus "threefold") holds that God is one God, but three coeternal consubstantial persons or hypostases. These being the Father, the Son (Jesus Christ), and the Holy Spirit - as "one God in three Divine Persons". The three Persons are distinct, yet are one "substance, essence or nature" (homoousios).

A church is a building used for Christian worship services and Christian activities. The earliest identified Christian church is a house church founded between 233 AD and 256 AD. The word church also describes a body or assembly of Christian believers, while "the Church" refers to the worldwide Christian religious community.