Being is a transcendental and any actual/potential thing must share that attribute in some form. For example, the genus “cats” may have various species within its genus but each one analogically share in common that same genus (or type) even if they’re not univocally the same as the other species within that genus. Being is similar to a “genus” but unlike “cats” which can be subsumed under another genus (e.g, it’s a mammal), Being cannot be subsumed under any other genus. Hence, why it must be considered a “transcendental” under scholastic metaphysics. Following this line of reasoning, we can apply this to God, who is understood as “Pure Act”. That which is purely actual cannot have potential in its being, either in its composition or in its essence (refer to the essence/existence, act/potency, and matter/form distinctions) by definition. Thus its being (essence) would simply be its act of being (existence) and its form would simply be its form.
If being had any divisible parts in its being then its parts could not equal the sum: cats + dogs =/= being in a univocal sense. To suggest this to be the case would require us to think that all beings are equivalent to cats + dogs, which would also require us to think that cats and dogs are univocally the same being. This is self-evidently absurd. Such things are sets of “a” genus, not the sets of “the” genus. Now, the perfection of being is pure actuality, which no other thing can possibly posses. And anything other than Pure Act cannot exist apart from it giving an act of being to such potential beings. Furthermore, in accord to the law of proportionate causality, a cause cannot give what it does not have. In which case, every effect that God produces must be analogically present in Himself. Thus, if God is Being, then He cannot be composed of any parts or He could not be God.