In mainstream Christianity, the Holy Spirit is one of the three divine persons of the Holy Trinity who 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). The New Testament has much to say about the Holy Spirit. The Holy Spirit’s presence was especially felt following the ascension of Christ, although not to the exclusion of an early presence as attested by the Old Testament and throughout the New Testament. The Christian theology of the Holy Spirit, or pneumatology, was the last piece of Trinitarian theology to be fully explored and developed, and there is thus greater theological diversity among Christian understandings of the Spirit than there is among understandings of the Son and the Father. Within Trinitarian theology, the Holy Spirit is usually referred to as the “Third Person” of the triune God—with the Father being the First Person and the Son the Second Person.