Here's an interesting principle.
- If a human being x exists in worlds w1 and w2 and x's lifetime occupies the same times in the two worlds, and at every time t in this lifetime, x is no better off in w2 than in w1, then x is no better off in w2 than in w1.
Are there good candidates for strictly posthumous benefits? Well, of course, such things as having a joyous afterlife might be examples, but those examples wouldn't be helpful for arguing that there is an afterlife.
What about such things as someone's posthumous keeping of a promise or fulfillment of a request, or maybe a writer's gain in reputation after death? I am not sure the benefit is strictly posthumous in these three cases. If you keep your promise to me, you bring it about that a promise that won't be kept wasn't made to me. And so, arguably, by keeping your promise you make me have been better off at the time the promise was made. Likewise, if my request was fulfilled or my writing gained in reputation, I did not request or write in vain, so I was better off at the time of the request or writing.
Posthumous forgiveness by you of my wrongdoing against you might be a better example of a strictly posthumous benefit. It seems that the benefit of being forgiven accrues not at the time of wrongdoing but at the time of forgiveness. If so, then that posthumous forgiveness would be a strictly posthumous benefit gives an argument for an afterlife.
But perhaps the benefit of forgiveness somewhat accrues at the time of wrongdoing. Maybe one is worse off there and then if one does a wrong that will never be forgiven. That sounds right to me. However, I don't think this accounts for the entirety of the benefit of being forgiven. Being forgiven removes guilt. However, the argument is now weakened. For the denier of an afterlife can claim that posthumous forgiveness only gives one the benefit of not having committed a wrong that won't be forgiven. It is definitely a benefit, but not as great one as forgiveness while one is alive.
I still think that consideration of posthumous benefits like those of forgiveness gives some evidence for an afterlife.