If rationality is subjective, then there’s no reasonable means by which to determine that a course of action is rational because there is no way to ensure that you yourself are being rational.
Exactly. Rationality is subjective, therefore, rationality only exists in the eye of the beholder. What is rational to one, can be irrational to another.
A Christian might find the concept of reincarnation totally irrational, but a Hindu would surely beg to differ.
I’m not a philosopher, so I don’t know if this is called relativism or what. It certainly has parallels in post-modernism’s concept of human beliefs being socially constructed, though.
We ascribe meaning because we must
We surely do, but isn’t that meaning also subjective? For example, I ascribe great meaning to a photograph of my deceased mother. It sits on my nightstand, and it helps me remember happy times I spent with her. The very same photograph is meaningless to virtually everyone else. If my house caught on fire, I would run in to rescue it, but no one else would.
Given that we know life has meaning, then the only rational conclusion is that we also know that there is something afterwards, even if we’re unwilling to consciously acknowledge that fact.
I dispute both of your arguments here. Life has meaning for certain, but that meaning varies tremendously among people. Different religious traditions, different societies, different times, all value life vastly differently (think about how native American life was valued by 19th century Americans, for example). Sometimes, life is given no value at all. The variation is such, that I don’t think we can say that we absolutely know life has any intrinsic meaning beyond what we give it.
And second, no, even if you value life tremendously, like my rabbi friend, you are not irrational for rejecting an afterlife. He states very bluntly, that if God created an afterlife, he would have given it a prominent role in Torah. Instead, the Torah teaches about how to treat other people, and commands the Jews to live it out. Again, he can in no way be described as a nihilist, and neither can thousands (millions?) of others who hold no belief in an afterlife. An afterlife is not a necessary part of religion.