Are you talking about an afterlife? If theism is true, does that also necessitate that there is life after death? Theism implies a higher power outside the "naturalistic/materialistic universe." If there is such a higher power, our actions in the "naturalistic/materialistic universe" besides affecting this world, also have some impact effect on the other, with or without the resurrection. However, theism to me also implies the immortality of souls. So yes, I may be using theism in a little bit more narrow way than you. But I think most religions have held in this principle of afterlife or some union with "God" (eg the Hindu concept of Brahmin). If you're not talking about an afterlife, then why is the idea of "eternal reverbration" more true of theism than naturalism. In a naturalistic world, our actions affect the future too. As I think I alluded to, in a purely naturalistic/materialistc universe the world is a closed system bound by the laws of time and space. So that, our actions with other people only have effects within those constraints. With theism and a belief in a higher power, as I have outlined it, the actions continue to reverberate (and are amplified) in eternity due to the immortality of individuals and the interactions between souls.