This isn't about hell or the End Times. Chill out. For Aquinas, it's really important that everything has its own particular end, by which he means a goal or a purpose. The end of physical but non-living things is to resemble God insofar as they participate in his properties, mostly just by existing. The end of a plant is to resemble God, particularly by existing and being alive. The end of animals is to resemble God by existing, being alive, and having some sort of consciousness. Everything exists to be caught up into God in accordance with its own particular thingness. A rock exists to be the rockiest rock it can be; a poodle to be maximally poodle-y, and your momma to be so fat that when she steps on the scales they say “to be continued...” Everything came from God, and everything has its end in God.
So, what is the distinctively human end?
This is complicated because human nature is complicated. On the one hand, humans are animals, and like animals we participate in God in existing, in being alive, and in having consciousness (One of my lecturers once got involved in an icebreaker game in which everyone had to answer the question, ‘What is your favourite animal?’ Being an Aquinas scholar, she was used to thinking of people as animals and so she replied, ‘My husband.’ Only later did it occur to her that the funny looks she got might be the consequence of implying that ‘My husband is an animal’ which means something rather different now than it would have for Aquinas). Humans are distinguished from the rest of the animals by the fact that they have reason and intellect, and are also distinguished by the fact that, where the end of the animals is in fulfilling their nature, the final end of human nature surpasses the intrinsic capabilities of human nature. Our end is in God, not just in participating in his qualities, but in contemplating his essence, in knowing and loving him, which is made possible by our reason but also by additional grace that takes us beyond what we’re naturally capable of. Our end is ‘the beatific vision’ which basically means ‘the happy vision.’
A note on reason
When we think about reason, we think about it in a post-Enlightenment world in which it often feels a bit like the sort of calculation a computer or a machine does, and see it as being fundamentally opposed to other things that make us human, like emotion. This isn’t quite the case for Aquinas, firstly because he doesn’t chop human nature up quite so completely, but also because for him, our emotions and gut instincts and all the things that make mathematicians feel a bit queasy are parts of us which can get caught up into reason and make it richer. The reason that reason gets us to God is because it starts us thinking about the world. We see all the stuff going on, and we start asking why and how and who and when, and eventually we want to know how it all started in the first place, and what we’re doing here, and how we should live, and whether it’s even worth it, and much as in Plato’s Symposium, the beauty of the world leads us in the end to Beauty itself, to God.