21 See then, henceforth I speak in all security of that which you have already understood; I am inculcating no unknown lesson, but am only conveying to you by recapitulation what you have already received. Now, of these three things, one only has been yet named and expressed; “Memory” is the name of one only of those three, yet all the three concurred in producing the name of this single one of the three. The single word “memory” could not be expressed, but by the operation of the will, and the understanding, and the memory.
The single word “understanding” could not be expressed, but by the operation of the memory, the will, and the understanding; and the single word “will” could not be expressed, but by the operation of the memory and the understanding and the will. What I promised, then, I think has been explained, that which I have pronounced separately, I conceived inseparably. The three together have produced each one of these, but yet this one which the three have produced has reference not to the three, but to one.
The three together have produced the word “memory,” but this word has reference to none but the memory only. The three together have produced the word “understanding,” but it has reference to none but the understanding only. The three together have produced the word “will,” but it has reference to none but the will only. So the Trinity concurred in the formation of the Body of Christ, but it belongs to none but Christ only. The Trinity concurred in the formation of the Dove from heaven; but it belongs to none but the Holy Spirit only. The Trinity formed the Voice from heaven, but this Voice belongs to none but the Father only.
Source: Sermons on Selected Lessons of the New Testament (New Advent)