[* I'm talking about "what Christians believe" here, and what I mean is what has been handed down as agreed-upon Christian doctrine from ancient times all the way up to the present; these things are shared by Roman Catholics, by Orthodox (Greek, Russian, or otherwise), by all sorts of Protestants, and even sometimes by other Christian groups outside of those broad families. These are truly ecumenical doctrines, and they are definitely not theological novelties. Speaking of these as what Christians believe isn't meant here to suggest automatically that someone who rejects some of these beliefs is therefore obviously not saved. They may very well be; they may be true disciples of Christ for all I know. But they don't hold to the historic orthodox faith, which is what I'm concerned with here.]
One thing that Christians have historically believed is the doctrine, or teaching, of creatio ex nihilo. That's a fancy Latin phrase meaning "creation out of nothing". Now, that doesn't mean that God took a big fistful of a stuff called 'nothing', and then poof! he turned it into a universe. That's crazy talk. What it does mean is this. The world has, in some sense, a beginning. Christians have often held that it had a first period of time to its existence, and in many respects it's gotten some strong scientific support from modern cosmology. So 'before' that beginning to the world, God was all there was. It's not like, pre-universe, there was God plus a lot of other stuff lying around. No, God is the only eternal reality, not God plus matter. There was nothing that just happened, lucky for God, to be there so that God could make stuff with it. When it came to creating the world, it was entirely God's decision for there to exist anything but God. If God had wanted, he was totally free to not create anything. And if he'd made that call, then eternally there would be only God - and that's it, because that's enough. (More on that in a moment.) God has no equals. Nothing but God's own self, or aspects of his own self, can be co-equal with God, because only God is a necessary being. And God did not need anything outside of himself in order to create. That's the main lesson we get from creatio ex nihilo - as opposed to the idea, which the early Christians rejected, of creatio ex materia, which would have meant that God had some matter existing eternally and independently alongside of God that he used to make the world. Like I said, there was no such matter; God made that, too. And what that teaches us is that everything that exists is dependent on God. Nothing is independent of him. We are dependent on God for our existence. God created us, and he is absolutely responsible for the fact that we exist; we are dependent and contingent and merely temporal, while God is independent and necessary and eternal. Like Christian philosopher Peter Van Inwagen said:
To say that God is the creator of all things besides himself is not to say that he formed them out of some pre-existent stuff, like the cosmic craftsman of the Timaeus. If there is a God, then there never was a chaos of prime matter that existed independently of his power and his will, waiting through an eternity of years for him to impress form on it. This could not be, for, if there is a God, nothing does or could exist independently of his will or independently of his creative power. God creates things from the ground up, ontologically speaking. His creation is, as they say, ex nihilo.1
Two other Christian philosophers have similarly noted:
For the author of Genesis 1, no preexistent material seems to be assumed, no warring gods or primordial dragons are present - only God, who is said to "create" (bara, a word used only with God as its subject and which does not presuppose a material substratum) "the heavens and the earth" (eth hassamayim we'eth ha'arets, a Hebrew expression for the totality of the world or, more simply, the universe). Moreover, this act of creation took place "in the beginning" (bereshith, used here as in Is 46:10 to indicate an absolute beginning). The author thereby implies creatio ex nihilo (creation out of nothing) in the temporal sense that God brought the universe into being without a material cause at some point in the finite past.2
So God did not need anything outside of himself to create, but Christians also believe that God did not need anything outside of himself for any other reason either - including his own happiness, or fulfillment, or for relationships. God is not just a God of love; God is essentially love. He doesn't need or require us, because everything needed for relationship, he has eternally had within his own being, within his self. What Christian teaching says that? Turns out it's a pretty famous one: the Trinity. That's right, the doctrine of the Trinity isn't just some academic thing that theologians talked about and argued over for ages simply to hear themselves talk or find something to fight over. It's actually important! It's important because, for one, it shows us that relationship isn't something that required creation. It's not as if God was forced to create because he got bored. God could never be bored because God's life is the eternally blessed communion of the Father with the Son and the Spirit, of the Son with the Father and the Spirit, and of the Spirit with the Father and the Son. And it's not as if God felt lonely; how could God feel lonely when he is, in his very character, relational within himself? So what this shows us is, first, that God does not need us for companionship or for anything else. God has everything it takes to be completely and perfectly fulfilled, and he had it eternally and necessarily. If he didn't, then he wouldn't be eternally and necessarily perfect, which is also something that Christians believe about God. What it also shows us is that God is and has always been perfect relationship in himself, from before the world began, and that when he freely chose to create a world, he created it in a way that reflects his nature. We were not made to be alone, because God is not alone. We were made to be in community because God himself is Community. And God created us so that he could enter into contingent and free relationships with us - not because he needed to, but because he freely chose to - and so that he can by grace bring us as contingent partners to share in the community of the Trinity. Think about that. God called us so that by grace we can share in the relationships that are from everlasting to everlasting, so that we can be brought into the communion of the Father and the Son and the Holy Spirit! This participation in God's inner life, as participants who have been freely invited by God from the outside, is a large part of what ancient Greek-speaking Christians - as well as Orthodox Christians today - call theosis, 'deification'/'divinization'. That doesn't mean that we become God by nature - we don't get to be omnipotent, for example, nor do we get to become the source of all things that exist, or uncreated, or anything like that - but it does mean that we get to share in some profound ways in who God is, as an act of God's grace.
Now, all those ancient heresies that some people who haven't done their history homework think were valid forms of Christian belief but were oh-so-brutally repressed by those evil orthodox Christians who wanted to control everyone? ("Help! Help! I'm being repressed!") Those heresies don't give you this. Take Sabellianism, for example. That's an ancient heresy, also known as 'modalism', that taught that there was really only one person who was God, and that when you saw the 'Father' and the 'Son' and the 'Holy Spirit' in the Bible, this was just one divine person playing a bunch of different roles, like a guy with a puppet in each hand, pretending to have real conversations with them - but really, there's only one center of consciousness there, only one person, so there's no actual relationship going on. That's what a lot of Christians mistakenly think the Trinity is, but it isn't. Jesus was not praying to himself, he was praying to a distinct person: his Father. There are real relationships between them. If Sabellianism were true, then if God hadn't created, he might well have gotten lonely; he could not have had a relationship unless he created something to have one with. And in a real sense, that would mean that God would have been in need, and therefore not perfect. And that God could never have a rich enough inner life to invite us to share in; it would forever be closed to us. Not only would that God be too small, so would the salvation he offers. Sadly, modalism of some form is still with us today in the teachings of the United Pentecostal Church International as well as other Oneness Pentecostal groups, as well as in the personal beliefs of a lot of well-meaning Christians who haven't been taught well enough to know better.
Or take Arianism, for example. This was the heresy that the Council of Nicaea fought in the year 325. Arius, a church elder from Alexandria in Egypt, taught that the Father was the uncreated God, but that the Son was a lesser created god who wasn't eternal like the Father. And because the Son was of a different essence from the Father, the Son couldn't know the Father perfectly; there are some things about God, in other words, that even Jesus doesn't get, and so he can't perfectly reveal God to us. What's worse, only God is eternally perfectly good; all other created agents can choose to fall away. What this meant for Arius was that it's theoretically possible for Jesus to rebel against the Father just like Satan did - and if he does, then say goodbye to your salvation. (Oh, and also, since only the one true God is worthy of worship, if you've ever worshipped Jesus, you're an idolater. Congratulations! The early Arians actually tried to hang on to some form of worship of Jesus, but they ultimately couldn't escape the logical consequences of their views.) And if you believe that Jesus is the Word/Reason (Logos) and Wisdom of God, like the New Testament says and the church recognized, then to believe that Jesus is a created being means that God was once totally speechless, irrational, and foolish until he created Jesus. This is why early Arians had to resort to saying that God had an uncreated Word/Wisdom as well as a created word/wisdom, and that Jesus is the second one instead of the first.
Note that this all means that Arianism has nothing better to offer than Sabellianism here. God in eternity is still a solitary individual without a robust inner life of relationship and community, and he can't bring us into a divine community by grace. Add to that all the other problems with what Arius was teaching, and you can see why the church had to make a big fuss about it! So when the Nicene Creed said that the Son is "of the same essence as the Father", using the Greek word homoousios, they were safeguarding what they knew about Jesus and his eternal relationship to the Father, as well as the fact that the church had always worshipped the Father and the Son and the Holy Spirit (which is why when some heretics decided to concede when it came to Jesus and go after the Spirit instead - they were called 'Pneumatomachians', or 'those who fight against the Spirit' - they didn't get any further). Sadly, forms of Arianism are still with us today among Jehovah's Witnesses, Christadelphians, and a bunch of other sects, as well as probably a lot of uninformed Christians in the pews.
Sabellianism can't give you a God of eternal community. Neither can Arianism. Neither can other anti-Trinitarian ideas such as tritheism, which is any belief that the Father, Son, and Holy Spirit are three totally separate gods. Aside from the fact that this is obviously not what Christians have ever believed, each of these 'gods' would be incomplete without the others. None of them has relationship within his own self, his own divine essence. And so while they might be able to invite you to join their society and become a peer, it isn't their inner life that you're entering into. And there's also the possibility that, unless they are necessarily always in agreement, they'll fight and break off their relationships - and then where would we be? But if they are necessarily in agreement, and if they do share an inner life, and if they're all co-eternal and mutually related, then really they aren't three gods but one God, just as the doctrine of the Trinity says.
None of these false theologies, these heresies, can give you what orthodox Christianity can. And in my opinion, not only is the doctrine of the Trinity better grounded in what the Bible teaches, but the doctrine of the Trinity is simply more beautiful, more promising, and more meaningful. There really are practical implications to believing in these things; don't let anyone try to tell you otherwise. Only the doctrine of the Trinity, coupled with creatio ex nihilo, speaks to you about one God who has no needs, who exists eternally as Community - which means that our relationships on earth are a reflection of something divine. And only these doctrines affirm God's true freedom, independence, and unique eternity in creating. And only these doctrines offer you the hope of being invited to participate by grace in the Community of the Father and the Son and the Holy Spirit, in which they eternally participate by nature.
2 J. P. Moreland and William Lane Craig, Philosophical Foundations for a Christian Worldview (Downers Grove, IL: InterVarsity Press, 2003), 554.