Grace

The "Grace" is supposed to be is an energy source for the Angels. It gives them their powers and it can only be taken away by the Angel Blades/Swords as they are able to "steal" the Grace in a sense by absorbing it's energy. However there's a way to remove the Grace without dying in the process.

What the Grace is an energy for:

 * Regeneration.
 * Healing ability.
 * Never can age.
 * Invisibility.
 * Not needing to consume food or drink to survive along with not having to sleep.
 * Sedation.
 * Angel Radio.
 * Telekinesis.

What it is not for:

 * Supernatural perception.
 * Short ranged Telepathy.
 * Teleportation (Wings control this)
 * Alchemy.

By Angel Blade/Sword:
If the Grace is removed by the form of an Angel Blade or Sword the Grace is completely removed, or more accurately, absorbed from their bodies, leaving them dead in the end process.

By one's own process:
You can manually remove your Grace from yourself without killing you, but in the process you go through excruciating pain to the point where it feels as if your life force is being taken from you. In the end process you are left as a normal human being, but you are able to see the Supernatural and use short ranged telepathy. You can also die by old age and normal human diseases.

To get it back is an unknown, unspoken process.