When we talk about the simplicity of God, we have in mind the idea that God cannot be broken down into parts. In order to wrap our minds around how God has revealed himself to us, we talk about his attributes, e.g. mercy and severity. But “God’s attributes are God.” While it is true that God loves—and that’s an attribute of God—it is also true that God is love. This is something that can’t be said about us. You may love, but you are not love. Additionally, because God’s attributes are God, each attribute both “summarizes the whole of God’s being” and “qualifies the others.”
God’s simplicity therefore assures us that he is not capricious, acting in severity in one moment but in mercy the next. There is a consistency in his interactions with creation, especially with mankind, that flows from his simplicity. If God is wisdom, power, holiness, justice, righteousness, truth, goodness, etc. at all times and in all ways, then when he reveals himself to us in Scripture and when we commune with him in our daily lives, we don’t have to wonder which part of God we will get. The truth is that we will see the one true God in Scripture always and in every way, and we will commune with the one true God always and in every way.
This is why it is so devastating to try to overemphasize (or even eliminate) certain attributes. That imbalance distorts our understanding of God in important ways. J.I. Packer notes that this happened in liberal theology when it created a doctrine of God that presented him as “a celestial Santa Claus,” whose severity was extracted and only mercy was left. But then Packer highlights the devastation of this functional denial of God’s simplicity:
Yet the Santa Claus theory carries within itself the seeds of its own collapse, for it cannot cope with the fact of evil. … This [collapse] was inevitable, for it is not possible to see the good will of a heavenly Santa Claus in heartbreaking and destructive things like cruelty, or marital infidelity, or death on the road, or lung cancer. The only way to save the liberal view of God is to dissociate him from these things and to deny that he has any direct relation to them or control over them; in other words, to deny his omnipotence and lordship over his world.
To the contrary, Douglas Kelly reminds us why upholding the truth of how God has revealed himself, including his simplicity in all the fullness of his attributes, moves us in a constructive direction: “[God’s] holiness requires the punishment of sin, but at the same time his heart is full of the most tender love for sinners.”
These two things, God’s severity against sin and mercy towards sinners, which are simultaneously and completely true at all times, only make sense at the cross, where God declares himself both just and the justifier of the one who has faith in Jesus Christ. At the cross, severity and mercy meet. In Christ, there is forgiveness of sins without downplaying the seriousness of sin. This is only possible because of God’s simplicity.
To end with a point of practical application, when you fall into sin, you need not wonder if you will get justice or mercy when you come to him in prayer; what you will encounter is the one true God in the fullness of all of his attributes. “We may call upon him through Christ with complete confidence that all of God is with his children for their good.”
 Joel R. Beeke and Paul M. Smalley, Reformed Systematic Theology (Wheaton, Illinois: Crossway, 2019), 626.  Beeke and Smalley, Reformed Systematic Theology, 626.  J. I. Packer, Knowing God, 20th anniversary ed. (Downers Grove, Ill: InterVarsity Press, 1993), 160.  Beeke and Smalley, Reformed Systematic Theology, 635.