The doctrine of the Trinity is encapsulated in Matthew 28:19, where Jesus instructs the apostles: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit."
The first Christians, like the Jews before them, were fiercely monotheistic, willing to die horrible martyrs’ deaths in the Coliseum—being slain by gladiators, devoured by wild animals, crucified, or tied to a stake and turned into human torches—rather than concede the existence of any other gods.
This adamant insistence on monotheism is taken directly from the teaching of the Bible. Thus, in John 17:3 Jesus addresses his Father, saying, "And this is eternal life, that they know you—the only true God."
The early Christians were quick to spot new heresies. In the third century, Sabellius, a Libyan priest who was staying at Rome, invented a new one. He claimed there is only one person in the Godhead, so that the Father, the Son, and the Holy Spirit are all one person with different "offices," rather than three persons who are one being in the Godhead, as the orthodox position holds.
Certain groups, notably the Mormons, have committed the error of saying that God the Father has a body, and have thus become anthropomorphites, people who say that God has a human form.
In recent years, this form of doctrinal decay has also set in among certain segments of American Evangelicalism, most notably in the Pentecostal Word Faith movement. Evangelicals such as Finnis Dake, Jimmy Swaggart, Kenneth Copeland, and Benny Hinn have all (temporarily or permanently) bought into the idea that the Father has a body.
"Time is not our own, and we must give a strict account of it."
~ Toribio Mogrovejo, of noble birth, educated at the finest schools, law professor at the University of Salamanca, Spain; who became a tireless missionary, baptizing nearly 500,000 as Archbishop of Lima, including St. Rose of Lima, St. Francis Solano, and St. Martin of Porres. Canonized in 1726.