Explaining the theology behind Q&A 25 of the Heidelberg Commentary, Zacharias Ursinus has a lengthy discussion of Trinitarian theology, covering the basic theological affirmations and denials, but also parsing out the more difficult aspects of the logic and language. The balance between pastoral language and intent in the Catechism and then scholastic detail in the Commentary provides a nice illustration of the range of Ursinus’ gifts (supposing, of course, that recent scholarship is correct in dismissing Olevianus’ authorship).
To begin his explanation of the terms “essence” and “person,” Ursinus gives a basic summary of the Trinity using all the traditional language:
We may now readily perceive the difference between the Essence of God, and the Persons, subsisting in the divine essence. By the term, Essence, we are to understand, in reference to this subject, that which the eternal Father, Son, and Holy Ghost are considered, and declared to be, singly and absolutely in themselves, and which is common to the three. By the term, Person, however, we are to understand that which the three persons of the Godhead are considered and declared to be individually and relatively, or as compared with each other, and which they are according to the mode of existence peculiar to each. Or, we may define Essence as the very being of God — the very, eternal, and only Deity — whilst the term Person refers to the mode, or manner, in which the being of God, or the divine essence, subsists in each of these three. God the Father is that Being who is of himself, and not from another. The Son is that self-same Being, or essence, not of himself but of the Father. The Holy Ghost is in like manner the self-same Being, not of himself but from the Father and the Son. Thus the Being, or divine essence, of the three persons of the Godhead is one and the same in number. But to be of himself, or from another — from one, or from two; that is, to have this one divine essence of himself, or to have it communicated from another — from one or from two, expresses the mode of existence which is three-fold and distinct; to wit, to be of himself, to be begotten or generated, and to proceed; and hence, the three persons which are expressed by the term. Trinity.
The sum of this distinction between the terms Essence and Person, as applied to God, is this: Essence is absolute and communicable—Person is relative and incommunicable. This may be illustrated by the following example: It is one thing to be a man, and another thing to be a father; and yet one and the same is both a man and a father; he is a man absolutely and according to his nature, and he is a father in respect to another viz: to his son. So it is one thing to be God, and another to be the Father, or Son, or Holy Ghost; and yet one and the same is both God, and the Father, or the Son, or the Holy Ghost; that in respect to himself, this in respect to another.
Following this, Ursinus gives a lengthy examination of the logic behind this confession, what exactly the terms mean and how they relate to one another. The essence is the deity considered as quality, the divine nature. The persons are the mode of that essence’s existence, irreducible, distinct and always in relation to one another. Some key features stand out, namely traditional role of the eternal generation of the Son (as contrasted against any unique “Calvinistic” modification), a rejection of the persons being understood merely as relations, the simplicity and infinity of the divine nature, and the dissimilitude between the Trinity and anything in creation.
Addenda. The essence of a man who begets another is communicated to him who is begotten, but the person is not communicated; for he that begets does not bring forth himself, but another distinct from himself. The son, therefore, is not the father, nor the father the son, although both be real men. So in like manner the eternal Father hath by eternal generation communicated to the Son his essence, but not his person — that is, he begot not the Father, but the Son; neither is the Father the Son, or the Son the Father, although each is very God. Yet, although there is this resemblance, there is at the same time a great difference in the manner in which the divine essence, being infinite, and the human, being created and finite are communicated to another, which difference is to be carefully observed; for, first, in men, in the father and the son, the essence is as distinct as the persons themselves — the father and the son are not only two persons, but also two men distinct in essence. But in God, the persons are distinct, whilst the essence remains common, and the same; and therefore, there are not three Gods, but the Son is the same God in number which is the Father and the Son. Secondly, in persons created, he that begets doth not communicate his whole essence to him that is begotten, for then he should cease to be a man, but only a part is made over to him that is begotten, and made the essence of another individual distinct from him who begets. But in uncreated persons, he that begets or inspires, communicates his whole essence to him that is begotten, or that proceeds; yet so that he who communicates, retains the same and that whole. The reason of both differences is, that the essence of man is finite and divisible, whilst that of the Deity is infinite and indivisible. Wherefore, the eternal Father, Son, and Holy Ghost, constitute the one true God ; and yet the Father is not the Son, or the Holy Ghost ; neither is the Holy Ghost the Son; that is, they are one God — not three Gods, but three persons subsisting in one Godhead.
This distinction of essence and person is, therefore, to be observed, that the unity of the true God may not be impaired, or the distinction of persons be taken away, or something else be understood by the term person, than the truth which God’s word declares. Therefore these cautions are to be diligently observed:
1. That person, in relation to this subject, never signifies a mere relation, or office, as the Latins are wont to say, Principas personam tueri, to preserve the person of the prince, as formerly Sabellius falsely taught; much less does it signify he countenance or visible shape, representing the form or gesture of another; in which sense a stage-actor may play off the person of another, as Servetus of late years sported and trifled with the word person; but it signifies a thing subsisting truly distinct from others to whom it has a relation and respect, by an incommunicable property; that is, it signifies that which begets, or is begotten, or proceeds and not the office dignity, or rank of him that begets, or is begotten, or proceeds.
2. That the persons do not constitute something abstracted or separated from the essence which they have in common, nor that the essence is any fourth thing separate from the three persons; but each of them is the entire and self-same essence of the Divinity. But the difference consists in this, that the persons are each distinct from the other, whilst the essence is common to the three.
3. Concerning the word essence, it is also to be observed, that God or the Deity, or the divine nature, has not the same respect to persons as matter has to form, for the reason that God is not compounded of matter and form. We cannot, therefore, correctly say, that the three persons are or consist of one essence. Neither is it as the whole in respect to the parts, because God is indivisible; therefore, we cannot correctly say that the person is a part of the essence, or that the essence consists of three persons; for every person is the whole divine essence. Neither is it as the general to the particular, because essence is not the genus of the three persons, nor is person a species of essence. But God is a more common name, because the essence of the Deity is common to the three persons, and therefore may be affirmed of each of them. But the names Father, Son, and Holy Spirit, are not applied in the same general way, because the persons are truly distinct, so that we cannot predicate the one of the other. We may, therefore, correctly say, God or the divine essence is the Father, is the Son, and is the Holy Spirit; also, the three persons are one God, or in one God; likewise, they are one and the same essence, nature, divinity, &c.; and again, that they are of one and the same essence, nature, &c. Yet, it cannot be properly said, that they are of one God, because there is no one of these persons that is not himself whole and perfect God. Wherefore the divine essence is in respect to the persons as that which is communicated in an extraordinary manner is in respect to those things with which it is common. There is, however, not a similar or exact example of communication in any thing created.
~Commentary on the Heidelberg Catechism, pg. 130-132