The doctrine of the Divine Unity is a truth of natural religion; the doctrine of the Trinity is a truth of revealed religion.
The various systems of natural theism present arguments for the Divine existence, unity, and attributes, but proceed no further.
They do not assert and endeavor to demonstrate that the Supreme Being is three persons in one essence.
It is because this doctrine is not discoverable by human reason, that the Christian church has been somewhat shy of attempts to construct it analytically; or even to defend it upon grounds of reason.
The keen Dr. South expresses the common sentiment, when he remarks that “as he that denies this fundamental article of the Christian religion may lose his soul, so he that much strives to understand it may lose his wits.” Yet all the truths of revelation, like those of natural religion, have in them the element of reason, and are capable of a rational defense.
At the very least their self-consistence can be shown, and objections to them can be answered. And this is a rational process. For one of the surest characteristics of reason is, freedom from self contradiction, and consonance with acknowledged truths in other provinces of human inquiry and belief.
It is a remarkable fact, that the earlier forms of Trinitarianism are among the most metaphysical and speculative of any in dogmatic history. The controversy with the Arian and the Semi-Arian, brought out a statement and defense of the truth, not only upon scriptural but ontological grounds.
Such a powerful dialectician as Athanasius, while thoroughly and intensely scriptural-while starting from the text of scripture, and subjecting it to a rigorous exegesis-did not hesitate to pursue the Arian and Semi-Arian dialectics to its most recondite fallacy in its subtlest recesses.
If any one doubts this, let him read the four Orations of Athanasius, and his defence of the Nicene Decrees. In some sections of Christendom, it has been contended that the doctrine of the Trinity should be received without any attempt at all to establish its rationality and intrinsic necessity.
In this case, the tenets of eternal generation and procession have been regarded as going beyond the Scripture data, and if not positively rejected, have been thought to hinder rather than assist faith in three divine persons and one God.
But the history of opinions shows that such sections of the church have not proved to be the strongest defenders of the Scripture statement, nor the most successful in keeping clear of the Sabellian, Arian, or even Socinian departure from it.
Those churches which have followed Scripture most implicitly, and have most feared human speculation, are the very churches which have inserted into their creeds the most highly analytic statement that has yet been made of the doctrine of the Trinity.
The Nicene Trinitarianism is incorporated into nearly all the symbols of modern Christendom; and this specifies, particularly, the tenets of eternal generation and procession with their corollaries. The English Church, to whose great divines, Hooker, Bull, Waterland, and Pearson, scientific Trinitarianism owes a very lucid and careful statement, has added the Athanasian creed to the Nicene.
The Presbyterian churches, distinguished for the closeness of their adherence to the simple Scripture, yet call upon their membership to confess, that “in the unity of the Godhead there be three persons, of one substance, power, and eternity; God the Father, God the Son, and God the Holy Ghost. The Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father; the Holy Ghost eternally proceeding from the Father and the Son.”
The treatise of Augustin upon the Trinity, which is here made accessible to the English reader, is one of the ablest produced in the patristic age. The author devoted nearly thirty years of his matured life to its composition (A.D. 400 to 428).
He was continually touching and retouching it, and would have delayed its publication longer than he did, had a copy not been obtained surreptitiously and published. Source.
William G. T. Shedd. For the full essay by Shedd, go here.