The Apostle John never tires of exhorting Christians to love one another, and the passage before us now is one of the most forceful exhortations that he provides. He grounds it not simply in the commandment of the Lord Jesus (e.g., John 13:34–35) but in the very nature of God—“God is love.” In saying this, John is teaching us that love belongs to the very essence of God. God is not loving in any contingent sense, as if He could have been otherwise. He is essentially and necessarily loving. Indeed, we may say that He is love itself. He is the original fountain from which every other instance of love flows.
But in God, this is true of all His attributes. What God is, and all that He is, He is essentially and necessarily. He could not have been otherwise. As God is loving, so also He is just, good, wise, merciful, etc. Or, if we want to express His attributes as nouns, God is justice, goodness, wisdom, mercy, etc., even as He is love. Unlike human beings or angels who may be loving or unloving, good or evil, wise or foolish, merciful or cruel, God is necessarily loving, as He is necessarily “infinite, eternal, and unchangeable, in his being, wisdom, power, holiness, justice, goodness, and truth” (Westminster Shorter Catechism 4).
Theologians have considered the essential unity of all God’s attributes under the heading of His simplicity, though the idea here might seem to be anything but simple. The point is not that this idea is easy to communicate or comprehend (after all, it is God we are talking about), but that God’s attributes do not dwell in Him as parts that are distinct from one another or from Him. God is not a composite being made up of separate and different components. Even the distinct persons of the Trinity are not to be considered as parts or components of the one true God. Each person is fully God, with all of Them sharing a single essence—an essence that is simple, not composite.