I am reading Dogmatic Theology by Willliam Shedd this year. I came across this paragraph on the necessity of divine simplicity:
Although trinal, divine essence is simple, not compound. In this respect, the unity of the finite spirit resembles that of the infinite. The spirit of man is not composed of two substances. It is homogeneous. It is all spirit. A material unity is complex, being composed of a variety of elementary substances. Hence, there are varieties of matter, but not of spirit. By reason of its incomplexity and simplicity, divine essence is indivisible. Not being made up, as matter is, of diverse parts or properties, it cannot be divided or analyzed into them: “The nature of the Trinity is denominated simple, because it has not anything which it can lose and because it is not one thing and its contents another, as a cup and the liquor, or a body and its color, or the air and the light and heat of it” (Augustine, City of God 11.10). Shedd, W. G. T, Dogmatic Theology, p223.