Lateran Council IV, 1215

Father, and the Son, and the Holy Spirit, and these three are one" [I John 5:7], and immediately is added: "And there are three who give testimony on earth, the Spirit, the water, and the blood, and these three are one" [I John 5:8], as is found in certain texts.

432. We, however, with the approval of the sacred Council, believe and confess with Peter Lombard that there exists a most excellent reality, incomprehensible indeed and ineffable, which truly is the Father, and the Son, and the Holy Spirit, at the same time three Persons, and anyone of the same individually; and so in God there is Trinity only, not a quaternity; because any one of the three Persons is that reality, namely, substance, essence or divine nature, which alone is the beginning of all things, beyond which nothing else can be found, and that reality is not generating, nor generated, nor proceeding, but it is the Father who generates, the Son who is generated, and the Holy Spirit who proceeds, so that distinctions are in Persons and unity in nature. Therefore, although "one is the Father, another the Son, and another the Holy Spirit, yet they are not different" ¹ but what is the Father is the Son and the Holy Spirit entirely the same, so that according to the true and Catholic Faith they are believed to be consubstantial. For the Father from eternity by generating the Son gave His substance to Him according to which He Himself testifies: "That which the Father has given to me is greater than all things" [John 10:29]. But it cannot be said that He (the Father) has given a part of His substance to Him (the Son), and retained a part for Himself, since the substance of the Father is indivisible, namely, simple. But neither can it be said that the Father has transferred His substance to the Son in generating, as if He had given that to the Son which he did not retain for Himself; otherwise the substance would have ceased to exist. It is clear, therefore, that the Son in being born without any diminution received the substance of the Father, and thus the Father and the Son have the same substance, and so this same reality is the Father and the Son and also the Holy Spirit proceeding from both. But when Truth prays to the Father for His faithful saying: "I will that they may be one in us, as we also are one" [John 17:22]: this word "one" indeed is accepted for the faithful in such a way that a union of charity in grace is understood, for the divine Persons in such a way that a unity of identity in nature is considered, as elsewhere Truth says: "Be . . . perfect, as also your heavenly Father is perfect" [Matt. 5:48], as if He said more clearly, "Be perfect" in the perfection of grace "as your heavenly Father is perfect" in the perfection of grace, that is, each in his own manner, because between the Creator and the creature so great a likeness cannot be noted without the necessity of noting a greater dissimilarity between them. If anyone, therefore, shall