No sober Trinitarian would take upon him to say precisely to what degree the distinctions in the Godhead extend. It is generally supposed, however, that the term person approaches the nearest to the Scriptural idea of any term that could be applied to this subject; yet those who use and contend for this term, in opposition to that of three names or three properties, do not mean to suggest that the distinctions in the Deity are in all respects the same as between three persons among men. The latter have no necessary connexion or union with each other, so as to denominate them one. It is highly probable that there is nothing in creation perfectly analogous to the mode of the Divine subsistence; and therefore nothing by which it can be fully conceived. And what if this should be the case? Where is the wonder that there should be something in God peculiar to himself, in the mode of his existence, which we cannot comprehend? If Socinians would but modestly consider the weakness of the human understanding, they would not decide so peremptorily on the other hand concerning the unity of God, as that it must needs be personal, or not at all. If it be too much for us to say with exactness to what degree the distinction reaches, is it not also too much for them to decide upon the precise kind and degree of union which is necessary to denominate the great Creator of the world—the one God?

The doctrine of a Trinity in unity is evidently a doctrine of pure revelation, and could never have been discovered by the mere light of nature. But, by comparing Scripture with itself, we may plainly perceive that the Divine unity is not a unity of person. Though there are three in the Godhead who are dignified with the same incommunicable titles of Jehovah, God, and Lord—possessing the same attributes and perfections, and entitled to the same worship arid adoration—yet the Scriptures do not exhibit a plurality of Deities, but teach us that Jehovah our God is one Jehovah. The obvious conclusion is, that these three are one God, and that the Scripture doctrine of unity is of more persons than one in the Godhead. The following passages, among many others, are very full to this purpose:—

“Go, teach all nations; baptizing them in the name of the Father, of the Son, and of the Holy Spirit.—There are three that bear record in heaven; the Father, the Word, and the Holy Spirit, and these three are one.—I am one that bear witness of myself.—The Father that sent me beareth witness of me.—It is the Spirit that beareth witness.—And the Holy Spirit descended in a bodily shape like a dove upon him; and a voice came from heaven which said, Thou art my beloved Son, in whom I am well pleased.—When the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me.—Now I beseech you, brethren, for the Lord Jesus Christ’s sake, and for the love of the Spirit, that you strive together with me in your prayers to God for me.—Through him (that is, Christ) we both have access by one Spirit to the Father.—Praying in the Holy Spirit, keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life.—The Lord direct your hearts into the love of God, and the patient waiting for Christ.—The grace of our Lord Jesus Christ, the love of God, and the communion of the Holy Spirit, be with you all.”

On reading these and similar passages, together with a great number of others which teach the proper Deity of Christ, we conclude that in a mysterious way, far above our comprehension, there are in the Divine unity three subsistencies; and as the New Testament constantly represents each of these three as bearing personal names, sustaining personal offices, and performing personal acts, we think ourselves warranted in accounting them three Divine persons.

Socinians, however, object to the doctrine of the Trinity on account of its being incomprehensible: and Dr. Priestley denies that the first teachers of Christianity taught any “mysterious doctrines, or doctrines in their own nature incomprehensible;”* and insists upon the necessity of “considering in what manner three persons are one God, upon the general principle that every proposition, before it can be believed, must be understood in some sense or other.”

The first preachers of Christianity taught the self-existence of God: Rev. 1:4, “Grace be unto you, and peace, from him who is, and who was, and who is to come.” But the self-existence of God is allowed by Dr. Priestley himself to be so much of a mystery that “he does not understand the manner of it.” He can here distinguish between things which are above reason and things contrary to it. “Though it be above our reason,” he says, “to comprehend how this original Being, and the cause of all other beings, should be himself uncaused, it is a conclusion by no means properly contrary to reason.” Now, why might not an atheist demand of Dr. Priestley an account of the mode or manner how God himself can exist, upon the general principle, “that every proposition, before it can be believed, must be understood in some sense or other?” Why should not this general principle apply to the manner in which God always existed, as an uncaused Being, as well as to the manner in which three persons are one God? And if it be proper to distinguish between things above reason and things contrary to it, in the one case, why not in the other?

The truth is, it is not necessary that every thing contained in a proposition should be clearly understood, in order to our being rationally convinced that such a proposition is true. We ought not to deny every thing we cannot comprehend; otherwise a man born blind would reason right when he forms this syllogism: We can only know the shape of different substances by feeling them; but it is impossible to handle them at a distance; therefore, it is impossible to know the shape of different bodies which lie beyond our reach! A blind man, by the concurring testimony of all about him, may be convinced that the figure of different bodies may be clearly ascertained by sight, though we cannot handle them. But when convinced of this on the ground of testimony, he never can be made to conceive how this is true. It is therefore a fundamental maxim, in all true philosophy, that many things may be incomprehensible and yet demonstrable, that though seeing clearly be a sufficient reason for affirming, yet not seeing at all can never be a reason for denying.

When it is affirmed that in the Godhead there are three, and that these three are one God, it has been objected, not only that the doctrine is incomprehensible, but that the terms themselves involve a contradiction; to this it might be replied, that if the Divine Being were affirmed to be three in the same sense in which he is said to be one, the objection would be valid; but the contradiction here is only a seeming one, and is no other than what appears in other propositions concerning the Divine Being, which are also true. Suppose it were affirmed that it is possible for God to do evil, and yet that it is impossible he should do evil: this would involve an apparent contradiction; and if the two branches of the proposition were to be understood in the same sense of possible and impossible, the contradiction would be real. But to say that it is not naturally impossible for God to do evil, were he so inclined, is only affirming what is necessary to his being a free agent, and so of being virtuous or holy; and to say that it is morally impossible for God to do evil is only ascribing to him that perfection of holiness which constitutes the true glory of his character. So to affirm that the centre and surface of the globe are exceedingly remote, and yet so exceedingly near as to be equally the central point of infinite space, is an apparent contradiction, and yet demonstrably true. That the remotest periods of time are alike the centre of infinite duration is also a most evident truth, and yet a caviller might object that the terms of these propositions involve a contradiction; it is like saying that two points may be one, and that one may be two. Yet, opposite as the terms may appear, the truth of the proposition is not at all affected by them, but rests on the strongest demonstration.

Fuller, A. G. (1988). “The Doctrine of the Trinity,” Miscellaneous Tracts, Essays, Letters, etc. The Complete Works of Andrew Fuller: Expositions—Miscellaneous (J. Belcher, Ed.; Vol. 3, pp. 707–709). Sprinkle Publications.