A subject so great and so much above our comprehension as this requires to be treated with trembling. Every thing that we can think or say, concerning the ever blessed God, requires the greatest modesty, fear , and reverence. Were I to hear two persons engaged in a warm contest upon the subject, I should fear for them both. One might in the main be in the right, and the other in the wrong; but if many words are used, they might both be expected to incur the reproof of the Almighty: “Who is this that darkeneth counsel by words without knowledge?”

The people of Israel were forbidden to break through the bounds which were set for them, and to gaze on the visible glory of Jehovah. The Bethshemties, for looking into the ark were smitten with death. Such judgments may not befall us in these days; but we may expect others, more to be dreaded. As the gospel is a spiritual  dispensation, its judgements, as well as its blessings, are chiefly spiritual. Where men have employed themselves in curiously prying into things too high for them, they have ordinarily been smitten with a blast upon their minds and upon their ministry.

There is a greater importance in the doctrine of the Trinity than commonly appears on a superficial inspection of it; chiefly, perhaps, on account of it affecting our views of the doctrine of the person and work of Christ; which doctrine, being the foundation of which the church is built, cannot be removed without the utmost danger to the building.

It is a subject of pure revelation. If the doctrine be not taught in the oracles of God, we have nothing to do with it; but if it be, whether we can comprehend it or not, we are required humbly to believe it, and to endeavour to understand so much as God has revealed concerning it. We are not required  to understand how the three are one; for this in not revealed. If we do not consider the Father, Son, and Spirit as being both three  and one in the same sense, which certainly we do not, then we do not believe a contradiction. We may leave speculating minds to lose themselves and others in a labyrinth of conceits, while we learn what is revealed, and rest contented with it.

Excerpt: Letters on Systematic Divinity: Letter IX – On the Trinity

Fuller, Andrew,  The Works of Andrew Fuller. Edinburgh: Banner of Truth Trust, 2007.