3 Before We enter upon this subject, it will be both desirable and useful to say a few words about the Mystery of the Blessed Trinity. This dogma is called by the doctors of the Church "the substance of the New Testament," that is to say, the greatest of all mysteries, since it is the fountain and origin of them all. In order to know and contemplate this mystery, the angels were created in Heaven and men upon earth. In order to teach more fully this mystery, which was but foreshadowed in the Old Testament, God Himself came down from the angels unto men: "No man bath seen God at any time; the only begotten Son, who is in the bosom of the Father, He bath declared Him" (John i., 18). Whosoever then writes or speaks of the Trinity must keep before His eyes the prudent warning ofthe Angelic Doctor: "When we speak of the Trinity, we must do so withcaution and modesty, for, as St. Augustine saith, nowhere else are moredangerous errors made, or is research more difficult, or discovery morefruitful" (Summ. Th. la., q. xxxi. De Trin. 1 L, c. 3). The danger thatarises is lest the Divine Persons be confounded one with the other in faith orworship, or lest the one Nature in them be separated: for "This is theCatholic Faith, that we should adore one God in Trinity and Trinity inUnity." Therefore Our predecessor Innocent XII, absolutely refused thepetition of those who desired a special festival in honour of God the Father.For, although the separate mysteries connected with the Incarnate Word arecelebrated on certain fixed days, yet there is no special feast on which theWord is honoured according to His Divine Nature alone. And even the Feast ofPentecost was instituted in the earliest times, not simply to honour the HolyGhost in Himself, but to commemorate His coming, or His external mission. Andall this has been wisely ordained, lest from distinguishing the Persons menshould be led to distinguish the Divine Essence. Moreover the Church, in orderto preserve in her children the purity of faith, instituted the Feast of theMost Holy Trinity, which John XXII. afterwards extended to the Universal Church.He also permitted altars and churches to be dedicated to the Blessed Trinity,and, with the divine approval, sanctioned the Order for the Ransom of Captives,which is specially devoted to the Blessed Trinity and bears Its name. Many factsconfirm its truths. The worship paid to the saints and angels, to the Mother ofGod, and to Christ Himself, finally redounds to the honour of the BlessedTrinity. In prayers addressed to one Person, there is also mention of theothers; in the litanies after the individual Persons have been separatelyinvoked, a common invocation of all is added: all psalms and hymns conclude withthe doxology to the Father, Son, and Holy Ghost; blessings, sacred rites, andsacraments are either accompanied or concluded by the invocation of the BlessedTrinity. This was already foreshadowed by the Apostle in those words: "Forof Him, and by Him, and in Him, are all things: to Him be glory for ever"(Rom. xi., 36), thereby signifying both the Trinity of Persons and the Unity ofNature: for as this is one and the same in each of the Persons, so to each is equally owing supreme glory, as to one and the same God. St. Augustine commenting upon this testimony writes: "The words of the Apostle, of Him, and by Him, and in Him are not to be taken indiscriminately; of Him refers to the Father, by Him to the Son, in Him to the Holy Ghost" (De Trin. 1. vi., c. 10; 1. i., c. 6). The Church is accustomed most fittingly to attribute to the Father those works of the Divinity in which power excels, to the Son those in which wisdom excels, and those in which love excels to the Holy Ghost. Not that all perfections and external operations are not common to the Divine Persons; for "the operations of the Trinity are indivisible, even as the essence of the Trinity is indivisible" (St. Aug., De Trin., I. 1, cc. 4-5); because as the three Divine Persons "are inseparable, so do they act inseparably" (St. Aug., i6.). But by a certain comparison, and a kind of affinity between the operations and the properties of the Persons, these operations are attributed or, as it is said, "appropriated" to One Person rather than to the others. "Just as we make use of the traces of similarity or likeness which we find in creatures for the manifestation of the Divine Persons, so do we use Their essential attributes; and this manifestation of the Persons by Their essential attributes is called appropriation" (St. Th. la., q. 39, xxxix., a. 7). In this manner the Father, who is "the principle of the whole God-head" (St. Aug. De Trin. 1 iv., c. 20) is also the efficient cause of all things, of the Incarnation of the Word, and the sanctification of souls; "of Him are all things": of Him, referring to the Father. But the Son, the Word, the Image of God is also the exemplar cause, whence all creatures borrow their form and beauty, their order and harmony. He is for us the Way, the Truth, and the Life; the Reconciles of man with God. "By Him are all things": by Him, referring to the Son. The Holy Ghost is the ultimate cause of all things, since, as the will and all other things finally rest in their end, so He, who is the Divine Goodness and the Mutual Love of the Father and Son, completes and perfects, by His strong yet gentle power, the secret work of man's eternal salvation. "In Him are all things": in Him, referring to the Holy Ghost.
Source: Divinum Illud Munus (Vatican.va)