There is a great need for biblical moderation in the study of the doctrine of the Trinity. What do I mean by that? I mean that it is possible, especially in reaction to one particular doctrinal error or another, to swing the pendulum to the oppose end of the spectrum, so to speak, and in an overreaction to error, embrace error of another kind. The idea of having biblical moderation in our trinitarianism is that we be careful to regulate our doctrine by the scriptures in such a way that we avoid errors in the extreme of any direction.
When we consider the errors of Arianism and Modalism, for example, we may note that they in many respects are the mirror opposites of each other, with orthodoxy lying between them. Both heresies embrace part of the truth, while rejecting another. Modalism affirms that the persons of the Trinity are of the same divine nature; as does orthodoxy. Yet Modalism also says that the three persons are really one person, whereas orthodoxy teaches that they are three distinct persons. Compare that with Arianism, which agrees with orthodoxy that the persons are distinct, and yet also says that They are distinct from one another in their nature, as well.
We can also observe that both heresies have historically been opposed to each other; Arianism really began as an over-reaction to modalism in the third century. In the early fourth century, Arius came along, and started the Arian controversy by accusing Alexander, bishop of Alexandria, of teaching modalism. And in the Arian controversy, none were so strongly opposed to Arianism as modalists such as Marcellus. “Homoousias”, which became the watchword against Arianism, was previously a word mainly associated with modalists such as Sabellius and Paul of Samosata.
It can also be argued that the eventual success of semi-modalism over orthodoxy, in large part, was due to a similar overreaction to Arianism, which has essentially remained a theological boogyman, especially to western Christianity, ever since the Arian controversy. And so we see a bit of a pattern emerge: Modalism arises in the second century, and is quickly opposed as heresy by the orthodox. In the rush to avoid modalism, and extreme weariness of it, Arianism is born, really as an overreaction to Modalism. The orthodox then oppose Arianism (with the help of the modalists), and the focus of what is perceived as the greatest theological danger to be avoided is placed upon Arianism instead of Modalism. Not surprisingly, overreaction again, this time in response to Arianism, leads to the widespread embrace of semi-modalism.
In rejecting semi-modalism, then, we must be careful not to make the same mistake (which is so easily made) as many who came before us, by overreacting to semi-modalism in such a way that we embrace some error in the other direction, such as Arianism or Unitarianism. Rather than being reactionary, and seeking to get as great of a distance as possible between ourselves and semi-modalism, we must be as moderate as we can, and be very intentional to return to the happy via media of orthodoxy, without swerving off of it to one side or the other.
The line between correct trinitarian doctrine and false trinitarian doctrine is often a relatively fine one. I hope this has been illustrated by the above discussion of Modalism and Arianism as mirror heresies; each has significant points of doctrine in common with what scripture teaches. The most dangerous errors are not those which are the most radical, and most distant from the truth, but those which are closest to the truth, and most easily blend in with it. These errors are the most dangerous because they are the most difficult to detect as being error.