De Moor V:30: The Utility of the Doctrine of the Trinity
We additionally observe, that the doctrine of the Trinity is also deservedly commended by its Utility for the practice of true Piety, against the same Remonstrants, who think that no particular practical Uses are able to be derived from the doctrine of the Trinity, beyond those that the doctrine of God and His attributes in general furnishes. Thus indeed the Apologia Remonstrantium, page 52b, has it: “The Uses of the doctrine of God have already been set down, even indeed with sufficient prolixity. But now in this chapter it is only asserted, that that God has revealed Himself under three persons or hypostases. Does anyone, except one mad, feign new Uses here? For whatever Uses will be feigned here, those will not be feigned under any other concept than under the concept of Deity. A Person, as He is a person, is not able simply to furnish a Use, except insofar as the person is divine.”
Now, MARESIUS, Systemate Theologico, locus III, § 60, in the Notes, observes, that the Scope/Goal of the Remonstrants in this is, that they might deny the Article concerning the Trinity to be fundamental doctrine, inasmuch as the knowledge of it does nothing to further practice. Although, says he, in that assertion they are depending upon a false hypothesis, as if all Theology were only Practical, having nothing of the theoretical, whereby our intellect might no less be perfected by a knowledge of the truth, than our will by the practice of the good. Indeed, for legitimate practice a distinct knowledge of the legitimate object of our practice and of our whole Worship is required beforehand. By which very thing this doctrine is already known to me Most Useful for the true Practice of Piety: just as our AUTHOR enumerates here this and other Uses. For, with other Accommodations set aside, concerning the avoidance of Solitude, Genesis 2:19; Ecclesiastes 4:9-12; concerning mutual Communication, Hebrews 13:16; etc.: he especially observes that this Doctrine leads us:
α. To known and acknowledge humbly the Incomprehensibility of the Divine Majesty, and the Imperfection of our Wisdom, Proverbs 30:2-4, on which passage see those that I have commended above in § 9.
β. To the true and appropriate Worship of the true God, as it was said in § 29. We learn here to venerate the Father Electing, Adopting, Justifying; the Son Redeeming; the Spirit Sanctifying, Preserving, Consoling.
γ. Then also to the πληροφορίαν, full assurance, of Faith and Hope, with God not only determining Salvation, but with the same also acquiring in the Son, and applying in the Spirit: which we apprehend is only able to be done in an appropriate manner, with the Trinity known and acknowledged: compare VOETIUS’ Disputationum theologicarum, part I, pages 477-487; LILIENTHAL’S Oordeelk Bybelverklar, chapter IV, § 30-34, part 1, pages 277-285.
[Wherefore we all the more detest the blasphemies of the Socinians, when they dread not to call the Trinity Antichrist’s Idol, Geryon, Cerberus, etc.] See HOORNBEECK’S Socinianismum confutatum, tome I, book I, chapter IX, section II, pages 221-224, book II, chapter V, section I, pages 415, 416; ARNOLDI’S refutationem Catecheseos Racovianæ, on chapter I, de Cognitione Personæ Christi, question 26, § CIX, page 123, who then on question 39, pages 41, 42, where the Catechist by various petty reasonings attempts to show how our opinion concerning the Trinity is pernicious, 1. demonstrates the nullity of these arguments calumniously brought against us; and, 2. proves the eminent utility of our doctrine concerning the Trinity; and, 3. evinces how the error of the Socinians in this matter is pernicious, § CLXXXIX-CCII, pages 160-165.
[Indeed, that innocuous sayings, or more solid glosses, of some of Our Men have been wrested:] See the Calumnies and Logomachies that HEINRICH ALTING mentions and examines, locus III concerning God, Theologia elenctica nova, pages 85-90.
[Which we anathematize from the heart as overthrowing the Christian Religion:] Which the Remonstrants are unwilling to do; see Censuram Confessionis Remonstrantium, chapter III, § IV, page 57; Apologiam Confessionis Remonstrantium, pages 53a, b, 54a; TRIGLAND’S Antapologiam, chapter V, pages 78b, 80a, number 12, pages 89, 90. You may see the argument of this entire Chapter most amply handled by ZANCHI in the XIII books de Tribus Elohim, which constitute the whole of tome I of his Opera.
 In Greek mythology, Geryon was a three-headed, or three-bodied, giant of the island of Erytheia, in the western Mediterranean.  In Greek mythology, Cerberus was a three-headed, monstrous hound, guarding to the gates to the Underworld.