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St. Thomas, in his commentary on Boethius’ De Trinitate, brings forth the objection that “It seems that it is not the confession of the Catholic faith that Father, Son, and Holy Spirit are one God: because, as Boethius himself says, upon inequality there follows plurality of gods. But the Catholic Scripture, which is the head of the Catholic religion, as Augustine says in On the True Religion, states that there is inequality between Father and Son, as is evident from what is said in the person of the Son: the Father is greater than I (John 14:28). Therefore what is said is not the confession of the Catholic religion.” (DeTrin.C1.Q3.A4.2)
How St. Thomas answers this question gives us a hermeneutic for reading the New Testament in its statements about the relationship of the Father and the Son, which consistently reveals the Catholic doctrine of the Trinity and Incarnation.
He begins by distinguishing between three types of passages in which the relationship of the Father and Son are signified.
First, there are certain passages which signify the unity of substance of the Father and the Son and their equality. An example of this is John 10:30, The Father and I are one. These passages do not signify an absolute identity, or merely nominal distinction between the Father and Son (for, there is a real distinction), but signifies that the Father and Son have one substance (they share the same Godhead). These passages are used by Catholics in their defense against those who divide the essence, such as Tritheists, and those who wish to argue for an inequality of the persons, such as Arians. These passages are used by heretics, such as Sabellians, to deny the real distinction of the persons, but they are refuted by the other two types of passages.
Second, there are other passages which signify that the Son is less than the Father, such as the one quoted above, that the Father is greater than I. This does not signify a superiority of essence, rather, this is to be taken in the sense that the Father is greater than the Son according to his humanity, or, in the words of St. Paul, according insofar as He has taken on the form of a servant. (Phil. 2:7) These passages are used by Catholics in their defense against those who deny the truth of the incarnation. These are abused by heretics, such as the Arians, but such are refuted by the first type.
Third, there are those passages which do not signify equality or inequality, but procession, i.e., that the Son proceeded (as a mental word proceeds from the mind, c.f., Rationibus Fidei 3-4) from the Father, such as John 5:26, as the Father has life in himself, so he hath given to the Son also to have life in himself. This signifies the truth that the Son is Deum de Deo (God of God). First, that He is Deo (God), for, to “have life in Himself,” is only a property of the Divine nature, and, second, that such is eternally communicated to Him from the Father, for, this Divine attribute is “given to the Son.” These passages are used by Catholics in their defense of the faith against, 1. Those who deny that the Son is God, and 2. Those who deny that the Son was begotten.
Each of these passages verifies different aspects of the Catholic doctrine of the Trinity and Incarnation. Interestingly, each of these are taken from the gospel according to St. John, refuting any beliefs of contradiction or incoherence between the authors of scriptures. If any of these passages stand alone, they may be interpreted in a heretical sense, but, when combined, the only reading is that: there is One God (first type), the Father, Son, and Holy Spirit; The Son eternally proceeds from the Father, God from God, receiving from Him the unified essence of God (third type); The Son has, in time, taken on a human nature, and according to His humanity (second type) suffered, died, and was burried.