Perhaps Thomas Aquinas’ most important contribution to philosophy and theology is the concept expressed as “existence precedes essence.” Here is a very brief explanation.
Science can measure what a thing is, and therefore measure its attributes. We can measure how much it weighs, how long & tall it is, how fast it moves, what color it is, etc. If we measure every one of its attributes, we have defined its essence. Some say that a thing is merely a bundle of essences, so that if we explain all the attributes, we have explained the thing. In this view, everything about an object can be explained in color, weight, density, energy, etc.
But this is untrue. For adding up all the attributes never gets us to the object’s existence. The $100 dollar bill in my mind has all the attributes of a real bill, but it does not exist in my wallet. Thus Aquinas says that we must explain not only what a thing is, but also explain that a thing is. Being (latin: esse) is not explained by physical properties. A thing must have existence which is logically prior to its essence, thus “existence precedes essence.”
When we consider that everything we observe in the universe exists but does not have to exist (e.g., is contingent and not necessary), then we are left with a glaring question: Why is there something, when everything is unnecesssary?
Science can never answer this question, for it only deals with weight, color, speed, mass, energy, etc., and never deal with the question of why something exits. No less a mind than the brilliant Bertrand Russell punted when he concluded, “It’s just there.”
The existence of a baseball is not explained by measuring the baseball, and the universe is not explained by measuring the essence of the universe. That existence precedes essence is the basis for proving that God exists, is maintaining it to this day, and has reason and purpose for all of us and all that is around us.
For more, start in the Bible with Colossians 1:17.