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 hath seen God at any time; the only begotten Son, who is in the bosom of the Father, He hath declared Him" (John 1.18). Whosoever then writes or speaks of the Trinity must keep before his eyes the prudent warning of the Angelic Doctor: "When we speak of the Trinity, we must do so with caution and modesty, for, as St. Augustine saith, nowhere else are more dangerous errors made, or is research more difficult, or discovery more fruitful" (_Summ. Th._ 1a, q. 31. _De Trin._ 1. 1, c. 3). The danger that arises is lest the Divine Persons be confounded one with the other in faith or worship, or lest the one Nature in them be separated: for "This is the Catholic Faith, that we should adore one God in Trinity and Trinity in Unity." Therefore Our predecessor Innocent XII absolutely refused the petition of those who desired a special festival in honor of God the Father. For, although the separate mysteries connected with the Incarnate Word are celebrated on certain fixed days, yet there is no special feast on which the Word is honored according to His Divine Nature alone. And even the Feast of Pentecost was instituted in the earliest times, not simply to honor the Holy Ghost in Himself, but to commemorate His coming, or His external mission. And all this has been wisely ordained, lest from distinguishing the Persons men should be led to distinguish the Divine Essence. Moreover, the Church, in order to preserve in her children the purity of faith, instituted the Feast of the Most 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 facts confirm this truth. The worship paid to the saints and angels, to the Mother of God, and to Christ Himself, finally redounds to the honor of the Blessed Trinity. In prayers addressed to one Person, there is also mention of the others; in the litanies after the individual Persons have been separately invoked, a common invocation of all is added: all psalms and hymns conclude with the doxology to the Father, Son, and Holy Ghost; blessings, sacred rites, and sacraments are either accompanied or concluded by the invocation of the Blessed Trinity. This was already foreshadowed by the Apostle in those words: "For of Him,