Useful and Necessary Observations Drawn from the Rules Which Have Been
  Given
  1. Regarding the external (ad extra) works performed with respect to
  creatures when only one person is mentioned, or two, or when the entire
  Trinity is understood: Thus in the Creed the Father is called the Creator, but
  not to the exclusion of the Son or the Holy Spirit. For of the Son it is said
  in John 1:10, "He was in the world, and the world was made through
  Him;" and in Heb. 1:2, "Through whom also He made the worlds."
  And of the Holy Spirit it is said in Ps. 33:6, "By the Word of the Lord
  were the heavens made and by the Spirit of His mouth is all their power."
  Gen. 1:26, "Let us make man," speaks of all three persons in the
  Godhead at the same time.
  Thus providence and the sustaining and conservation of things are often
  attributed to one person, and yet it is the common work of the whole Trinity.
  Concerning the Father, Christ says in Matt. 6:26 and 10:29, "A sparrow
  does not fall to the earth without the will of your Father." Of the Son
  it is said in Heb. 1:3, "Upholding all things by the Word of His
  power." And of the Holy Spirit it is said in Ps. 104:30, "Send forth
  Your Spirit and they will be created, and You will renew the face of the
  earth."
  2. Gregory of Nazianzus notes that sometimes Scripture mentions the three
  persons, as "in the name of the Father, and of the Son, and of the Holy
  Spirit," and at other times as two persons, as in the exordia of the
  epistles, "Grace be unto you and peace from God our Father and the Lord
  Jesus Christ," and at still other times as one person, as at the close of
  the Pauline epistles, "The grace of our Lord Jesus Christ be with
  you," This way of speaking at times preserves the order of the persons,
  as in the preceding examples, and at other times is indifferent to the order,
  as when, the name of the Son is placed ahead of the name of the Father in
  2Cor: 13:14, "The grace of the Lord Jesus Christ, the love of God, and
  the communion of the Holy Spirit be with you all." Likewise when the name
  of the Holy Spirit is placed ahead of the name of the Son, as in Eph. 3:16-17,
  That you may be strengthened in the Spirit, so that Christ may dwell in your
  hearts by faith." That is to say, Scripture bears witness that the three
  persons and the entire Trinity are the one true God, and that each person is
  perfectly and in all respects that one true God. For when in John 10:30 the
  Sons says, " I and the Father are one ," He is not excluding the
  Holy Spirit. For in 1John 3:24 it is said, "By the Spirit we know that
  the Father and the Son remain in us." And 1 John 5:7, " These three
  are one."
  3. Scripture attributes one and the same activity sometimes to the Father,
  sometimes to the Son, and sometimes to the Holy Sprit, in order to show that
  the external works are common to all, that the three persons exist at the same
  time and work together at the same time. In Rom. 16:25 the Father establishes,
  in 1 Cor. 1:8 the Son confirms, and in Eph. 3:16 the Holy Spirit strengthens.
  Likewise, in James 1:17 one is enlightened by the Father, in John1:9 the Word
  (ho logos) enlightens every man, and in Eph. 3:7-9 the Holy Spirit enlightens
  all through the ministry. This observation is also by Gregory of Nazianzus.
  4. The Arians, in opposition to this position, raised 1 Tim. 6:15-16,
  "The Father alone has immortality, alone is powerful"; Rom. 16:27,
  "The Father alone is wise", Luke 18:19, "No one is good, except
  God alone", John 17:3, the Son says to the Father,".that they may
  know You the only true God." Therefore the Son is not true God, because
  the word "alone" is used exclusively of the Father. But the answer
  is truly and fundamentally based on these rules. For when the Deity is placed
  in opposition to idols, or to creatures, then the mention of one person does
  not exclude the others from being of the same substance (homoousia) with the
  Godhead. Nor does the mention of two exclude the third. But the exclusive
  aspect pertains to and is used over against only idols and creatures. Cyril
  makes this observation.
  Moreover, we must consider the certain and firm testimonies of Scripture on
  which this correct answer relies. For the authority of Cyril does not suffice
  in itself. Thus when the Godhead is described internally (intra sese), then
  Christ clearly rejects the exclusive concept, John 8:16, 28-29,; John 16:32b.
  Therefore it is manifest that the concept of exclusiveness which is alleged in
  the passages cited applies to the first rule and excludes only idols and
  creatures (to which it is opposed) and not the other persons of the Godhead.
  For Christ rejects this exclusive concept. Thus when it is said in John 17:3
  that the Father is "the only true God," the Son is not excluded, for
  it says in 1 John 5:20, "The Son is the true God," and in John
  16:15, "All things which the Father has are Mine," and in Matt.
  11:27, "No man knows the Father except the Son, and no one knows the Son
  except the Father." Nor is the Holy Spirit excluded, because in 1 Cor.
  2:11 it says, "The things which are of God no one knows except the Spirit
  of God." And just as in Paul the Son is not excluded, so in Matthew the
  Holy Spirit is not excluded. Thus in Rev. 19:12, "The Son has a name
  which no one knows except Himself," it is manifest that neither the
  Father nor the Holy Spirit are excluded, but only creatures. These last points
  are made by Augustine, Contra Maximin [MPL 42.743 ff.], and demonstrate the
  true basis for this fourth observation.
  5. The church in its worship sometimes makes specific mention of the three
  persons, sometimes of two, and sometimes of one; and yet always it directs its
  prayers to the one true divine essence and at the same time to all the
  persons. For with respect to us the three persons are at the same time and
  each individually the one, true, undivided God, so that when the dove
  descended, one can correctly say that this is the one true God and beyond Him
  there is no other God, as it says in John 14:9, "He who sees Me, sees My
  Father also." And again in v. 10, "I am in the Father, and the
  Father in Me." On this basis we can understand how the church directs its
  prayers sometimes to the Father, sometimes to the Son, and sometimes to the
  Holy Spirit. For it believes and confesses in its prayers not only that the
  three persons are the one true God, but that each person is not just a part of
  that one divine essence but rather is the entire divine essence, that is, the
  one true God, than whom there is no other God. For he who invokes one person
  above or beyond the others, as if that person were separate or individual,
  errs from the true God, as it is said in John 5:23, and John 8:54-55b. This is
  the point which has been made by our revered father and preceptor Dr. Martin
  Luther, De Ult. Verb. Davidis, Vol. 8, Jena ed. [Amer. Ed. 15.302-03].
  6. The persons are distinguished not only by internal differences, such as
  that one begets, another is begotten, the third proceeds, but also by external
  differences which have been noted particularly by reason of revelation and
  beneficial actions toward the church, as is evident in the definition of each
  person. For in the external works ("opera ad extra") the three
  persons are together and work together, and yet with a certain order and with
  the properties of each preserved, as Augustine says in "Contra
  Felicianum," 10 [MPL 42,1164], Note 1 Cor. 15:57. The fathers often used
  the statement of Paul in Rom. 11:36, "For of Him and through Him and in
  Him are all things; to whom be glory and honor." For because the apostle
  is speaking of external works, he mentions the one eternal essence, "To
  Him be honor," not "to them." And yet, just as there is one
  essence without confusion of the persons, so this essence performs the
  external works in common for the three persons, without confusion, but hints
  at the difference of the persons - "of Him, in Him, and through
  Him." Therefore the external works, as our great Martin Luther sets
  forth, should be considered in a twofold sense. First, in the absolute sense,
  and thus without distinction, they are and are described as the works of the
  three persons in common. Second, in a relative sense, when they are considered
  as to the order in which the persons act, [we must consider] what the
  properties of each person are and what each person does in an immediate sense.
  Thus we must consider the work of creation, redemption, and sanctification in
  both the absolute and the relative sense.
  And in some way on the basis of this we can consider why sometimes only one
  person is mentioned, or why two, when the entire Trinity is understood. For
  example, "The Father, the fount of blessing," as the ancients say,
  and He is called the only Potentate, etc. likewise, "The Creator Father
  and the Son breathe the Holy Spirit into the hearts of the believers."
  Hence it is said in John 14:23, "I and the Father will come and make our
  abode with him." And in 1 John 3:24, "We know from the Spirit that
  the Father and the Son are in us."
  In summary, just as we believe in the unity of the essence and yet must not
  permit a confusion of the persons, so we must understand also this rule: the
  external works are common to the three persons, but in such a way that the
  differences and properties of the persons are not confused.
  All antiquity frequently made use of this observation in arriving at
  solutions of problems. But in worship this observation is absolutely
  necessary: for although the worship of the Deity is undivided, just as the
  external works are, yet the prayer of the church is especially for this reason
  separated from the prayer and worship of all other gentiles. For the church
  invokes the three persons without confusion, but takes into consideration the
  distinction and blessings peculiar to each of the persons.
  7. Of the names applied to the Deity some refer to the essence and some to
  the person. And between these categories there is a great difference, for
  example, the Father is God, eternal, omnipotent. Likewise the Son is God,
  eternal, omnipotent. The Holy Spirit is God, eternal, omnipotent. But we do
  not say there are three Gods, three eternals, three omnipotents, because these
  designations apply to the essence. And Augustine says, "So great is the
  power of each substance in the Trinity that what is said elsewhere concerning
  the individual persons (e.g., God is eternal, omnipotent), this, when
  referring to the whole, is not said in the plural but in the singular."
  Erasmus ridicules Athanasius because he does not wish to speak of three
  eternals, although he himself goes right on to say that the three persons are
  coeternal. But from these rules and fundamentals it must be understood how
  God-pleasing and useful is this care in speaking.
  Next we must observe this point, that the same designation can sometimes be
  applied to the essence and sometimes to the person. In accepting this concept
  there is no diminishing of the distinction. For example, the Son is not the
  Father, even if the term "Father" is used with reference to the
  person of the Son. For example, in Is. 9:6 the Son is called the "Father
  of the world to come" (Vulgate). The name "Father" is used with
  reference to the essence. And in the sequence for Trinity Sunday the church
  calls the Holy Spirit the "Father of the poor." Thus the Son is not
  the Holy Spirit when the term "spirit" is used with personal
  reference to Him. But because God is a spirit in essence, the Father is also
  spirit and the Son is spirit. Thus, these names refer to the essence:
  "The Father of mercies," 2 Cor. 1:3: "the Father of
  spirits," Heb. 12:9. So also in the Nicene Creed the terms are used in
  the personal sense, "God of God, Light of light," etc. And in the
  Lord's Prayer the term "Father" can be taken in the essential sense,
  because He is the antithesis to the creature and the prayer is directed to the
  entire Trinity. However, the term can also be taken in the personal sense in
  consideration of the benefits belonging to each of the persons, in accord with
  the statements of Paul in Rom. 8:15 and Gal. 4:6, "He sent the Spirit of
  His Son into our hearts crying Abba, Father." We worship in spirit, John
  4:23-24, and we call the first person the Father, because of the Son. And this
  conforms more closely to the apostolic form of the words.
  Perhaps many more examples can be piled up which have been discussed among
  the scholastics, but I wanted to select those which are most notable and which
  will commend themselves very well to this usage.
  But do not get the notion that these observations are foolish subtleties.
  But because God wills to be known, invoked, and proclaimed as He has revealed
  Himself, therefore we must make every effort to believe in a godly way
  concerning these great mysteries and speak reverently and soberly about them.
  And in this matter, we must imitate the diligence of the ancients by whom the
  truth of this article was fought for and defended in the face of heretics. For
  as Jerome says, "Heresy arises from the improper use of words."
  We can achieve this goal more easily if we keep these rules of the ancients
  before our eyes and have them in view when we speak about this article in a
  pious, reverent, and proper way.