“The doctrine of the Trinity is not so much a point among many as the very essence and compendium of Christianity itself. It not only presents a lofty and sublime subject of contemplation to the intellect, but furnishes repose and peace to the heart and conscience. To explain this mystery is not our province. All true theologians, who have trained and disciplined minds in the right school, whether in expounding positive truths or in combating erroneous views, have uniformly accepted it as their highest function simply TO CONSERVE THE MYSTERY and leave it where they found it, in its inscrutable sublimity, or, as the poet expresses it, “dark though excessive bright.” Leibnitz happily said, If we could bring it within the terms of any humanly constructed definition, it would be a mystery no longer. The zeal and erudition of the Fathers, accordingly, were mainly employed to retain and preserve the mystery.
“And when we look at the doctrine from the practical point of view, a belief of this great truth is absolutely essential to the Christian man and the Christian Church. Without it, Christianity would at once collapse. As this doctrine is believed on the one hand, or challenged on the other, Christian life is found to be affected at its roots and over all its extent. Every doctrine is run up to it; every privilege and duty hang on it. It cannot escape observation that scarcely a heresy ever appeared which did not, when carried out to its logical results, come into collision with the doctrine of the Trinity at some point. Through the whole history of opinion, the ever-recurring fact presented to us is that, however a man may begin his career of error, the general issue is that the doctrine of the Trinity, proving to an unexpected check or insurmountable obstacle in the carrying out of his opinions, has to be modified or pushed aside; and he come to be against the Trinity because he has found that it was against him.
“The attacks on the Trinity, menacing though they might be at the time, have commonly been the occasion of real benefit to the Church. The Church might have been less on the alert than was found to be imperatively necessary when asked, for instance, by the Sabellian to allow within her pale a mere modal distinction in the Trinity, as asked by the Arian to give a certain amount of liberty to such as questioned the or denied the supreme Deity of the second or third person of the Trinity. By varied discipline and experience, she has been schooled to apprehend the doctrine of the tri-personal God, or of the threefold personality in unity, as the most fundamental, vital, and practical of doctrines; that it forms the ultimate ground of every truth; that it is absolutely intertwined with the essential provisions of the gospel; and that the plan of salvation cannot be left standing entire, if this great doctrine, the keystone of the arch, is either loosened or displaced.
“The Church, accordingly, has always posted herself here as in the Thermopylae, where her last stand is to be made. She knew that, without this doctrine, the Creed would have no coherence, nor her members solid peace. The enlightened Christian neither expects nor wishes to find that which will not baffle his contemplation by its vastness, nor dazzle him by its splendor. Nay, the appeal to the ADORING WONDER of the finite mind becomes more powerful when its limited capacity fails to comprehend the theme in all its magnitude. The Christian Church, feeling that she has to believe what God has condescended to declare, and alive to the fact that there is no loyalty greater than the loyalty of the intellect, calls for the submission of the finite reason. Hence everyone feels the force of these beautiful words of Gregory Nazianzen in reference to the Trinity. In his sermon on Baptism he says: ‘I cannot think of the ONE but I am immediately surrounded with the slendour of the THREE; nor can I clearly discover the Three, but I am suddenly carried back to the One.’
“The objection to the Trinity on the ground of the unfathomable mystery, has been repeated in every successive age. And it may not be out of place to say that if there had been no mystery, an opposite objection might not improbably have emanated from the very same parties. Had there been no inscrutable doctrines beyond the sounding line of man’s reason, no profound mysteries in the revealed account of God’s Being, purposes, and works, - if such a thing were conceivable in a revelation communicated from God to man, - the objectors might have decried and depreciated it from a wholly different point of view as a stale, flat, and unprofitable message, which had nothing in it worthy of the claims which it made on men’s minds, because it had nothing beyond the discovery of the human understanding.”
The Doctrine of the Holy Spirit, George Smeaton (pp 4-7)