The idea of the rational soul, and the postulation of its immortality, I would think does have some rational grounding for an afterlife. How I would test for the coherency of an afterlife in comparison to that of no afterlife would be to use reason, and to think about the conditions necessary for either outcome to be true.
So to start would be to ask oneself what it would be like for no afterlife to exist, or so to say that, when one dies, then the consciousness by which we use to perceive the world, to access the rules of logic, and to even have these abstract talks about philosophy, would be gone, completely, or else to no longer exist, or to be nonexistent, most bluntly. So for the possibility of a lack of afterlife to be coherent, it would also be necessary for the concept of nonexistence to be coherent, or else if it is coherent, then it would be necessary to see whether it is coherent for something that exists to be able to not exist, such as our conscious mind that exists being able to fully not exist at the moment of death.
On the first part, nonexistence does truly seem to be an incoherent concept, because nonexistence would, if possible, be a state of affairs lacking all properties, both possible and actual. Nonexistence wouldn’t just simply apply to what certainty doesn’t actually exist, such as, say, pink elephants with blue ears and golden tusks, for even though such things certainly don’t exist actually, they could exist possibly, for they are applied with properties that, although unlikely and implausible together, are coherent and exist actually when apart from one another, such as the existence of elephants, the existence of colors such as red, blue, and yellow/gold, and the existence of animals having these colors of pigments imbued into their bodies. So, for something that doesn’t actually exist to possibly exist, it would need to possess attributes and properties that, independently of one another, actually exist. So if this logic holds, then for nonexistence to possibly exist, if would need actually existing properties attributed to it to be able to actually exist. But nonexistence is, by definition, a complete lack of anything that exists, which includes any actually existing properties that, brought together, can produce the actual existence of something that can possibly exist, such as nonexistence. By this logic, then nonexistence cannot possibly be a thing that exists, and if nonexistence cannot be something that exists, then this means that nonexistence cannot be a potential property that can be applied to things that currently exist, such as human minds, which do exist. Therefore, it seems incoherent to say that the mind, or the human consciousness, can end up becoming nonexistent.
But then one might say that, even if the mind, which exists, cannot undergo complete nonexistence, then it can still be radically changed after death, such as things like wood being radically changed when set aflame, and are turned into something else, like fire, smoke, and ash. In the same sense as material things can undergo complete change, so then, can immaterial things like the mind, also be changed radically by other things. But this has a problem: this seems like a category error, for it applies the rules of property changes as they relate to material objects to those that are immaterial objects, such as the mind. I can say, “Yes, material objects can undergo change. But even if they can, how does it follow that, therefore, immaterial objects like the mind can do so as well?” because this is the case, it would require the work of other skeptics to make the case that the rules of change that apply to material objects also apply to immaterial objects. And even if one could do so, they would need to make the case for what kinds of things can apply change to immaterial objects such as the mind. Can material objects do so? It doesn’t seem so at all, because it would be like saying that you can alter an idea by throwing a rock at it, or by changing the laws of logic by blasting them with a particle accelerator. But these ideas seem incoherent, because immaterial things like ideas and logic are not observable and don’t exist in the material reality, and so seem completely independent of causal influences like energy and matter, which are material. Therefore, material things don’t seem able to change immaterial things. So, if this is the case, then the only things that can change immaterial things would be other immaterial things, and so if death is a material things (Simply the full cessation of biological processes), then it cannot alter the immaterial mind, which could constitute the immortal soul.
But, if one of you is a functionalist, and so object that the immaterial mind can be significantly altered materially by arguing that the immaterial mind is merely an emergent property of the material parts of the brain coming together to form what is the mind, then you would need to be able to answer the hard problem of consciousness, which is how immaterial things can be produced by material ones. Because there is no strong answer to the question, then to make the claim that the mind is simply a product of the brain’s constituent parts would be to make an assertion, and not a substantive argument, against the argument which I’ve set forth. And if this is the case, then I shall simply invoke Hitchens’s razor, “What can be asserted without evidence can be dismissed without evidence.”
Putting all of this together, I think I’ve made a strong case as to why the lack of an afterlife is not a very reasonable concept, and so, if the lack of an afterlife is incoherent, or at least unfounded by sufficient reasoning, then one simply needs to make the case as to why the existence of an afterlife is coherent.
And I think it is more coherent because, unlike the problems of what nonexistence is, and the fact that nonexistence is necessary for the nonexistence of an afterlife, the existence of an afterlife is logically coherent by contrast, because not only is it a conceivable concept (Due to us making thousands of interpretations as to what it is throughout history), but it also complements the properties of the immaterial mind as being transcendent, because the afterlife would also be a transcendent domain if it exists (Unless, perhaps, reincarnation is true), and so, unless one could make a proof that the mind is actually made up of parts that aren’t transcendent, then the existence of a transcendent mind provides a strong grounds for the existence of a transcendent domain such as heaven, or at least reincarnation. Therefore, if we were to stack up the possibility that no afterlife exists, and that one does exist, and that no other possibility exists due to the law of excluded middle, then by demonstrating the seeming incoherency of nonexistence, the transcendence of the rational mind, and it compatibility with nonexistence versus existence after bodily death, then the possibility of an afterlife is the one that coheres better, even if it is, in truth, a seemingly unconventional possibility, and very weird. But then again, the world, as it is, is already extremely weird and baffling, so to dismiss the afterlife on the basis of it being weird is not, to me, any good evidence that it isn’t true.
Sorry if this response is very long, but I hope it answers your initial question sufficiently.