Doesn't the command by Matthew's Jesus to, "Go therefore, and teach all nations, baptizing in the name of the Father, and the Son, and the Holy Spirit" (Matthew 28:19) show the existence of a triune deity.
Answer: Matthew 28:19 states: "Go therefore, and teach all nations, baptizing in the name of the Father, and the Son, and the Holy Spirit." Although the Father, the Son, and the Holy Spirit are grouped together, this verse does not prove the existence of a triune deity. The verse merely indicates the author's belief that they are to be mentioned together during baptism. Each is thought to have a function in the initiation of the believer during the baptism ritual. Yet no doctrine of coequality among them is promulgated in this verse.
In the early period baptism was simply in "Christ" (Galatians 3:27) or in the name of Jesus (1 Corinthians 1:13, Acts 8:16, 19:5). The text in Matthew represents a later stage of development, but is still not trinitarian in meaning. The doctrine of the trinity is a still later development.