The simplicity of God is harmonious diversity in perfect unity, not reduced singularity.
God is one, but he is not reduced. God is one, but he is not the sum of parts. He is the entire of all that he is, always, and at the same time. To borrow from the famous Nazianzan, but modifying it with regard to his attributes: He cannot be contemplated in his attributes without the mind being drawn to the whole. He cannot be contemplated in the whole without the mind being drawn to contemplate the attributes. Hence God is without parts.
The old theologians said that within God there are distinctions, not parts. That is, we distinguish the attributes but we cannot separate them.God’s diversity is indivisible. His knowledge cannot be divorced from his holiness, nor his love from his justice, nor any of these from his immutability, and so on. Yet we distinguish knowledge from holiness from immutability.
Therefore, when John says “God is love”, he cannot mean to say that God is love with exclusion to justice, truth, knowledge, wisdom, jealousy, wrath, or eternity. He means that God’s love is just, true, knowing, wise, jealous, fierce, never-ending, unchanging, purely actualized, and without equal.
Therefore, when theologians speak of the Trinitarian relations as a perfect bond of love, they cannot mean love with exclusion to any of God’s attributes. That the “Father loves the Son” and “the Son loves the Father” is to say that with a just, true, holy, knowing, wise, jealous, fierce, never-ending, unchanging, purely actualized, and unequalled love does the Father love-know-embrace faithfully-deal justly-pursue relentlessly the Son, and the Son likewise does the same toward the Father, and so the Father and the Son for the Spirit and the Spirit for the Father and the Son.