The book of Genesis tells of how God created the earth in six days, as well as the first humans. He created Adam and Eve in his image as pure and uncorrupted beings. However, Lucifer seduced and slept with Eve within the garden, unbeknownst to her husband, infecting the human race with original sin. Eve had two sons, Cain and Abel, with the latter being the son of Adam and the former being the son of Satan. This led to God rejecting Cain and embracing Abel, leading to jealous rivalries between the two which would culminate in the first murder. Adam and Eve would go on to have another son named Seth, while Cain was branded a murderer and forced to walk the earth alone. Therefore, while the elves of the world stem from the seed of Seth as pure and uncorrupted, the human race would stem from the seed of Cain, tainted with the stain of original sin that he had passed down from his mother.This was all part of Satan's grand scheme to screw over God's divine plan by making humans susceptible to his manipulations.
Human souls contain life energy that sustains an individual during life, and are instrumental in the usage of magic. Demon magic focuses on releasing this life energy in massive amounts to create altering reality affects. This is done through the process of causing pain and suffering, such as a mass human sacrifice or a battle costing large amounts of lives. Directing and focusing this buildup of energy in the right way can cause a rift between the worlds of Hell and the mortal plane. These demonic spirits can cross the boundaries of Hell into our world through these rifts that open up between realms that allow passage.
The taint of original sin has allowed for a " back door " within humans, allowing them to be possessed by demons. Through demonic possession, a demonic spirit can take full control of an individual, suppressing their free will and trapping their soul within themselves. Being in our world gives them the capability to forcefully take control of a victim, making them a slave to the demon's machinations. The body of that human eventually withers and decays from the demonic energy, forcing the demon to hop from human to human, allowing him to continue his master's goals on earth.
It stands to reason that if a massive amount of built-up life energy is required to summon a demon from Hell, it should also allow for a demon to cross back over. However, as things turn out, there is a chink in this master plan. Apparently, crossing over from Hell into the mortal plane is a one way trip. A demon who passes through a rift connecting realities becomes trapped on the mortal plane, unable to travel back even while these rifts remain open or a new one is created. The only way for a demon to re-enter Hell is through an exorcism by an ordained priest, who exorcises a demon from a host, forcing them back into the pit. Aside from this, they are stuck bouncing from human to human unable to return home.
How can a barrier be constructed in a way that allows things to pass through them in only one direction?