More in the theology series...
1. Is Theology Practical?
2. Why Believe in God?
God as Creator
3. God as Other
4. God as All-Powerful
5. God as All-Knowing
6. God as Eternal
7. God as Spirit
8. Three in One
Probably no Christian belief is more mysterious than the Trinity. The Trinity of course is the firmly held belief, on the one hand, that there is only one God, coupled with the equally firmly held belief that God the Father is God, Jesus is God, and the Holy Spirit is God. The overwhelming majority of Christians throughout the centuries have believed that God exists as three persons, even though he is only one substance.
It seems very difficult to explain this belief in any detail without running into imbalance either on the one God side or the three persons side. And if we do fall into imbalance, there is sure to be a name from history for the imbalance. For example, if someone suggested that there were three gods, that is polytheism. If someone suggested there was only one God who changed his roles and functions throughout time, that is called modalism or Sabellianism.
There is a massive history here, and it is not clear how practical it is to know it except that it reinforces to us a sense that what Christians believe sometimes has as much to do with the church in history as it does with the Bible. There were options the Bible seems to leave open that were quite decisively shut closed in church history.
For example, some Jewish Christians of the early second century believed Jesus was the Messiah without believing he was divine (Ebionites, Nazoreans). Some New Testament interpreters see them in continuity with some of the earliest Christians, perhaps like the Judaizers. Acts 15 possibly treats such people as "in" at that time, but they would be "out" at least by the late second century.
Another early heresy was the idea that Jesus became God at his baptism, adoptionism. Then there was the idea that Jesus the divine spirit being never fully became human, Docetism (he only "seemed" to be human). We already mentioned modalism, which is the idea that God the Father in the Old Testament became God the Son in Jesus, who then ascended and returned to earth as the Holy Spirit.
Probably the biggest historical challenger to what we now consider to be orthodoxy was a heresy called Arianism. Arius at the beginning of the 300s believed that Jesus was the "firstborn of all creation" (Col. 1:15), that Christ was the first thing God created and the most pre-eminent thing that God created, but still part of the creation. At one point in the mid-300s, probably more Christians were Arians than were trinitarians. Again, you can see how Arius could argue his point from Scripture.
These ideas only became heresies at a certain point in time. That is to say, while it may be obvious to us today that they are false, it was not always obvious at the time. We might expect to see some of these thinkers in the kingdom, even though their beliefs were eventually condemned. Some only became heretics after they were dead.
Historically, it seems pretty clear that the doctrine of the Trinity developed in order to clarify several beliefs that seemed in tension with each other. So the belief that there is only one God is central to a Jewish understanding of the Old Testament, even if most of ancient Israel believed in the existence of other gods that were not legitimate recipients of worship. That is to say, the official practice of ancient Israel was "monolatry," the worship of one God, and they were "henotheists."
But by the time of the New Testament, such gods were thought of as demons (1 Cor. 10:20), and belief in one God seems firmly established within Judaism. Subordinates to YHWH could be reverenced as a function of worshiping him, such as bowing to the king or reverencing exalted angels that represented YHWH or bore his name. The New Testament strongly affirms that there is only one God (e.g., 1 Cor. 10:4-6; Eph. 4:6).
Yet the New Testament also considers Jesus to be God (John 1:1) as a distinct person, and some passages consider the Holy Spirit to be a distinct person (16:7-11) who is obviously God as well. So we have one God, but we have three persons who are God. This is basically what the idea of the Trinity, clarified at the Councils of Nicaea (325) and Constantinople (381), affirms. God is one in substance, but exists as three persons. This belief affirms everything that needs to be affirmed.
What is the practical pay-off? Probably the first that comes to mind from the history is a recognition of how much we owe to the church in the first four centuries for what we now believe about the Trinity. It may seem obvious to us today that the Bible teaches the Trinity, but it was not obvious to all the early Christians for several hundred years. Indeed, when the Reformation opened the door to a re-examination of catholic belief in the light of Scripture alone, we should not be surprised to find that some Protestants, like the Socinians, ended up rejecting the Trinity.
Even such revered Protestant names as John Milton (Paradise Lost) and Isaac Newton (laws of physics) seem to have rejected the classic view of the Trinity. John Wesley firmly believed in the Trinity, but believed individuals such as these would be in the kingdom.  Even today, there are "oneness Pentecostals" who are basically modalists who believe that the one God only has existed in history as one person at a time, and they would have an inerrantist view of the Bible. The long and short of it is that we seem to need more than the Bible alone in order to arbitrate such debates and that, as in the case of the biblical canon itself, one has to put some faith in the church to have certainty on the doctrine of the Trinity.
What other practical pay-off might the doctrine have? Certainly you hear lots of supposed practical insights allegedly drawn from the Trinity. Both egalitarians and complementarians sometimes refer to the Trinity as a basis for their beliefs.  Look, the egalitarian says, Christians have historically believed that the divine Son is not subordinate to the divine Father. The complementarian then responds with Scriptures where the (human) Jesus is subordinate to the divine Father. Both arguments seem rather arcane and probably unhelpful.
Others see the Trinity as an embodiment of love and relationship as an eternal attribute of God, with John 17 no doubt playing the key biblical role. The theologian Karl Barth structured his entire, voluminous theology on the structure of the Trinity. So much of the teaching we find right now on the Trinity seems quite speculative, especially considering how mysterious it is and how cautiously it came together in church history. So much thinking right now about it seems reminiscent of the layer after layer of paint added to Christianity in the medieval period.
In my view, the most practical view to take on the Trinity is to stick somewhat closely to its original form. We Christians affirm that there is only one God. We also affirm that Jesus is God and the Holy Spirit is God. We can talk of Father, Son, and Spirit as distinct persons while only affirming one God. To go much beyond this basic affirmation is to enter the often impractical world of speculation and how many angels can dance on the head of a pin.
 Egalitarians do not believe that it is God's long term will for wives to be subordinate to their husbands, even though God accommodated the culture of ancient times in parts of the Bible (e.g., Col. 3). Complementarians believe that God has built into the creation the subordination of wives to their husbands at least for the time prior to the coming of God's kingdom.
9. God as Love
10. God as Just
11. God as Unchanging