The Trinity or Trias is a Christian doctrine, stating that God is one Being who exists, simultaneously and eternally, as a mutual indwelling of three persons: the Father, the Son (incarnate as Jesus Christ), and the Holy Spirit. Since the beginning of the third century, the doctrine of the Trinity has been stated as three Persons in one God, all three of whom, as distinct and co-eternal persons, are of one indivisible Divine essence, a simple being. Closely linked with this doctrine is that according to which the Son has two distinct natures, one fully divine and the other fully human, united in a hypostatic union. Belief in the doctrine of the Trinity is known as Trinitarianism. Most denominations within Christianity are Trinitarian, and regard belief in the Trinity as a mark of Christian orthodoxy. For the concept, the word "Trinity" (in Latin, Trinitas) began to be used around the year 200. This Latin word means "the number three, a triad", an abstract noun formed from the adjective trinus (three each, threefold, triple). The Greek term used for the Christian Trinity, " Τριάς " ( Trias or Triados ) meaning "a set of three" or "the number three", and has given the English word triad. The Old Testament refers to God's Word, his Spirit, and Wisdom. These have been interpreted as adumbrations of the doctrine of the Trinity, as have been also narratives such as the appearance of the three men to Abraham in Genesis 18. Modalists attempted to resolve the mystery of the Trinity by holding that the Father, the Son and the Holy Ghost are merely modes, or roles, of God Almighty.