Last post in the section on God in my ongoing series giving some thoughts on theology in bullet points. I may have missed something, but I believe this concludes my section on God. I'll post all the links in a separate post in a moment.
G11. There is only one God, but God is three persons.
How can God only be one God and yet be three distinct persons? It is a mystery. Any attempt to explain this historic Christian belief is doomed to fall into something that has been considered a heresy at some point or another in Christian history. You might say that, at best, we might simply affirm everything we need to affirm and not try to figure out how it all fits together.
1. There is only one God.
This is a central pillar of the Jewish faith from which Christian faith emerged. Old Testament faith was of course more complicated in its Ancient Near Eastern context. The Israelites believed that Ba'al existed. They just didn't believe he was an appropriate god to worship nor anywhere near as powerful as Yahweh, the God of Israel.  The God of Israel was in fact God Most High, the Creator God, the one true God. The others were inferior beings who were nothing next to God. They were to put no god before Yahweh (Exod. 20:3).
We as Christians today would not even call such beings "gods." We might take Paul's lead and call them demons at best (1 Cor. 10:20)--perhaps even figments of the Canaanite imagination. The one true God is the sole Creator of the universe out of nothing.  He created all other spiritual powers, including those that were associated with other peoples in the Ancient Near East.
Paul puts it this way: "For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”), yet for us there is but one God, the Father, from whom all things came and for whom we live" (1 Cor. 8:5-6a). This is a consistent teaching of Scripture. Ephesians clearly states that, "There is... one God and Father of all, who is over all and through all and in all" (4:4, 6).
2. There are three persons who are the one God.
Here is where it gets complicated. Since the 300s, Christianity has affirmed not only that there is one God, but that God the Father is God, Jesus is God, and the Holy Spirit is God. As a creed from those early centuries puts it, we do not confuse the persons of the Trinity, this three-in-one. And we do not divide the one substance. 
As this creed puts it, "there is one person of the Father and one person of the Son, and one person of the Holy Spirit, but the divinity of the Father, Son, and Holy Spirit is one." All three are uncreated. All three are eternal. All three are all powerful. "The Father is God, the Son is God, the Holy Spirit is God. Yet there are not three gods, but one God."
The details of this belief were worked out in the first few centuries of Christianity as the Church tried to discern what the Bible indicated about God. So John 1:1 said that "the Word was God," meaning that Jesus was God. But did Jesus become God at his baptism or resurrection (adoptionism)? Was Jesus the first, most exalted of God's creation, but still part of the creation (Arianism)? Or perhaps Jesus was only similar in substance to God the Father (homoiousios) rather than "of the same substance" (homoousios).
On the other side of the equation, were the Father, Son, and Holy Spirit simply three different faces of God but not really three distinct persons (modalism)? After Christianity became a legal religion in the Roman Empire, the leaders of Christianity came together at a city called Nicaea in the year AD325 to try to hammer out some of these disagreements.  It would take the better part of a century, but by about the year 400, the current belief had become the commonly accepted one on the Trinity.
The common belief of Christianity ever since is that all three persons of the Trinity are distinct persons but that they are only one God.
3. This is a difficult concept, and it has always been tempting to modify it or give it greater clarity than we are in a position to understand. It is also a position that required Christians to answer questions that are not entirely clear in the Bible, and the different groups who argued in the first centuries all used Scripture to argue for their positions.
Even today, there is a tendency to go into great detail about relationships between the Father, Son, and Holy Spirit before the creation of the world. But in all of Scripture, the only passage to come anywhere close to saying anything along these lines is when Jesus speaks of the glory he shared with God the Father before the world began (John 17:5). In the Bible itself, we see the nature of Jesus' relationship with the Father while he is on earth but not so much before.
Knowing the tendency traditions have to grow, it is best for us to focus most on the Trinity in relation to the roles each play in Scripture and not to speculate too much about things we could not possibly comprehend. God the Father functions most in Scripture as the transcendent and sovereign Creator to whom all glory must ultimately go (e.g., Phil. 2:11). Jesus functions most in Scripture as the agent of God in the redemption of the world, the one in whom the world once again finds its intended meaning and purpose. The Spirit functions most in Scripture as the channel of God's presence and power on the earth.
In Scripture, God is three persons, even though there is only one God.
Next week: C.1 God created the universe out of nothing.
 The worship of one God while believing others exist is called "monolatry," and the position is called "henotheism."
 I am following the lead here of Richard Bauckham's description of Jewish monotheism at the time of Christ in Jesus and the God of Israel: God Crucified and Other Studies on the New Testament's Christology of Divine Identity (Grand Rapids: Eerdmans, 2008), 184. In addition to "creational" monotheism, he describes Jewish monotheism as "cultic monotheism" (meaning that only Yahweh was worthy of worship) and "eschatological monotheism" (only Yahweh would emerge victorious at the end of history).
 The so called Athansian Creed.
 The Roman emperor Constantine made Christianity a legal religion in AD313.