There's often a point in the crime drama where the bad guy tells the cop 'You and me, we're not so different really.' Well there are two baddies in the history of trinitarian theology who really aren't that different: Arius and Sabellius. Arius was the sub-ordinationist. He defined the One God such that Jesus could not fit in. Instead Jesus had to take His place under the One God. Sabellius was the modalist. He defined the One God such that Jesus was absorbed in, losing everything that made Him distinctly Jesus. Instead Jesus was just the mask that the One God wore occasionally. But you know - Arius and Sabellius weren't so different. They both had a doctrine of the One God that couldn't cope with Jesus.
It was Jurgen Moltmann who really nailed this in my thinking. Check out this quote from The Trinity and the Kingdom of God:
A pre-conceived doctrine of the One God means "Christ must either recede into the series of the prophets, giving way to the One God, or he must disappear into the One God as one of his manifestations." (p131)
Here are the errors of Arius and Sabellius - and Jesus gets either squashed down or squished in. The distinct Person of Christ will always lose out when 'the One God' is defined without Him. Arius will allow Him to be Jesus and not God, Sabellius will allow Him to be God and not Jesus. But fundamentally these errors are not so different because both prefer a pre-conceived 'One God' to Jesus.
This leaves us no option but to begin with a doctrine of God that expressly includes the mutual relations of Father and Son. Nothing else will allow Jesus to be Jesus and God. Moltmann helped me to see what was at stake in this. To begin with a definition of God that doesn't already include the distinct Personhood of the Son means either Arius's or Sabellius's error. And, at the end of the day, they're not so different.
This is why Moltmann says:
...the doctrine of the Trinity is not only the deification of Christ; it is even more the Christianization of the concept of God. God cannot be comprehended without Christ, and Christ cannot be understood without God. If we are to perceive this, we not only have to reject the Arian heresy; the Sabellian heresy must be dismissed with equal emphasis." (p131-132)
Christ will never fit into a 'God' defined without Him. We must begin with Him or else we will never honour Him properly. The errors of subordinationism and modalism are simply the result of falling off either side of the wrong horse. We must begin with Christ.