When a person dies, their soul doesn't ascend to the afterlife automatically. The soul builds a connection with its body while alive, and remains attached to that body in death. Through natural decomposition, the soul gradually loses its connection to the mortal realm. At some point, when the body decayed enough, the soul ascends to the next life where it will be judged by god. It will either be allowed into heaven or join the reincarnation cycle to be reborn and given another chance.
The ascension of the soul however, is not always guaranteed. While it is trapped in the body after death, the soul is protected from supernatural forces. However, There are invisible predators that prey on and consume vulnerable souls. These are called wraiths, which are angry spirits that have been unable to ascend properly and remain trapped on the mortal realm. These spirits are in constant pain, and seek to take out their agony and misery on the living by killing them. They are also given to consume other lost souls and grow in power. This has been common enough throughout history to be a real concern.
This is why funeral rites and proper burials are held as sacred, because they guaranty passage into the next life. Priests are used to help guaranty the process of ascension. Through performing the necessary funeral rites and given the deceased a proper burial, they protect the soul while it remains in the body from malignant forces. These are rites only chosen priests can perform, which involve complex magical rituals that are learned through years of study. This has given the church a strong presence, and has made them a powerful force in humanity. Religion and faith play an important role in daily life of people, and priests are held in high regard in their community. Otherwise interfering with a body (cremation, dismemberment, etc) is considered a great crime. If a body doesn't decay naturally, the soul may become more vulnerable.
Individuals who go off to war are given a special place in society. Warriors are given markings, or runes, by priests that are akin to magic tatoos. If they die in war, their souls ascend automatically without delay, so they can go into battle without fear. Being a soldier is a high honor, as is dying in war for your nation. I want to keep these special runes limited to soldiers so that only they can be given them. There needs to be a good reason for priests to not simply give them to everyone. What would be a non-malicious way to justify this?