The reason divine truth is light for angels is that angels are spiritual and not natural. Spiritual people see things from their sun, and natural people see from theirs. Divine truth is what provides angels with discernment; and discernment is their inner sight, which flows into their outer sight and produces it. So whatever is seen in heaven from the Lord as the sun is seen in the light. Since this is the source of light in heaven, it varies depending on the acceptance of divine truth from the Lord or (which amounts to the same thing) depending on the intelligence and wisdom angels participate in. This means that it is different in the heavenly kingdom than in the spiritual kingdom, and different in each community. The light in the heavenly kingdom looks fiery because the angels who are there accept light from the Lord as the sun. The light in the spiritual kingdom, though, is white because the angels who are there accept light from the Lord as the moon (see above, Section 118). Further, the light is not the same in one community as in another. It even differs within each community. People in the middle are in more light, and people round about are in less (see Section 43).
In short, to the extend to which angels are open to divine truth (that is, participate in intelligence and wisdom from the Lord), they have light. This is why heaven’s angels are called angels of light.
from Heaven and Hell, Section 128