In mainstream Christianity, the Holy Spirit or Holy Ghost is believe to be one of the three entities of the Holy Trinity which make up the single substance of God; that is, the Spirit is considered to act in concert with and share an essential nature with God the Father and God the Son (Jesus Christ). The Christian theology of the Holy Spirit, or pneumatology, was the last piece of Trinitarian theology to be fully explored and developed. For this reason, there is greater theological diversity among Christian understandings of the Spirit than there is among understandings of the Son (Christology) and of the Father. Within Trinitarian theology, the Holy Spirit is sometimes referred to as the "Third Person" of the Triune God - with the Father being the First Person and the Son the Second Person. This article also discusses understandings of the Holy Spirit by non-Trinitarian groups and some non-Christian groups who use the term as well.