Thesis 3 of the 24 Thomistic Theses:
Wherefore, in the exclusive domain of existence itself God alone subsists, He alone is the most simple. Everything else, which participates in existence, has a nature whereby existence is restricted, and is composed of essence and existence as of two really distinct principles.
Quapropter in absoluta ipsius esse ratione unus subsistit Deus, unus est simplicissimus, cetera cuncta quae ipsum esse participant, naturam habent quae esse coarctatur, ac tamquam distinctis realiter principiis, essentia et esse constant.
Commentary by P. Lumbreras, O.P.:
If there is any being, the actuality of whose existence—for existent means actual—is not received into the potentiality of essence, such a being subsists of itself, because it is perfection without limits; it is unique, because it excludes composition of any kind; it is the most simple Being: God. All other things, the actuality of whose existence is received into the potentiality of the essence, participate in existence according to the capacity of the essence, which limits in this way the actuality of existence. Essence and existence hold in them the place of Potency and Act in the existential order, and are two real and really distinct principles, which intrinsically constitute the compound, the existing being, in the order of existence.
[Summa Theologiae, Iª q. 50 a. 2 ad 3; Contra Gentiles, lib. 1 cap. 38 et cap. 52 et cap. 53 et cap. 54; Super Sent., lib. 1 d. 19 q. 2 a. 2; De ente et essentia, cap. 5; De spiritualibus creaturis, a. 1; De veritate, q. 27 a. 1 ad 8]
Essence limits existence as potentiality limits actuality.
For more info on the essence / existence distinction, see: