The Creed begins: "I believe in ONE God...", yet we talk about the Trinity: Father, Son, and Holy Spirit. How can this be?
Why just one? Because Christianity grew out of Judaism and we are bound by the Jewish affirmation in Deuteronomy 6:4:
Shema, Yisroel! Adonai Elohenu; Adonai Echod!
Hear, O Israel! The Lord is our God; the Lord is One!
What distinguished Israel from the surrounding polytheistic cultures was (and is) an uncompromising loyalty to the one God of Abraham, Isaac, and Jacob and their Covenant with HIM on Sinai.
The Hebrew of the Shema is short, clear, and concrete: ONE GOD! Christianity inherits that from Judaism.
Our other parent, however, was equally firmly founded in the Greek language (the earliest copies of the books of the New Testament are all in Greek, not Hebrew or Aramaic) and our thinking is grounded in Greek philosophy.
In the New Testament, Jesus says:
"I and the Father are one." (John 10:30)
"Whoever has seen me, has seen the Father." (John 14:9)
"Before Abraham was, I AM." (John 8:58.)
"That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us." (John 17:21)
And He talks about the Father sending a Comforter (John 14:16) who will teach (John 14:26) -- clearly another Person.
Greek theologians, starting from the basis: "Jesus said it -- I believe it." then go to the question: "How can this be?" Their answer (which we will see more of, later in this series) is that the Triune God is one ousia (essence/nature) in three hypostases (instances or persons).
The three Persons share the Godhead by a process called "perichoresis", which has a parallel on the computer on which I am writing this post.
The basic hardware now (Sep 2010) is a ~5 year old Dell SX270 PC, running the Fedora 13 distribution of the Linux operating system. Under Linux, a program called "Virtual Box" from Sun/Oracle runs the Windows 2000 operating system at the same time, and on the same hardware, as Linux. I switch back and forth with a mouse click.
The two "computer hypostases" share data and hardware, and can "talk" to one another, yet are completely individual. I can also add more instances -- other OSs operating in parallel on the same hardware -- subject to limits on timing and disk space. In an experiment, 3 instances running slowed the box down unacceptably.
The Holy Trinity, obviously, are not limited by finite numbers of clock cycles or storage space. ;-)
