Because Aqualord was destroyed during the Kōma War and the power of the other dragon lords were kept out of the area inside of the mazoku barrier, holy magic became unavailable in that area. After the destruction of Hellmaster Fibrizo, the barrier ceased to exist, and holy magic was once again castable. However, it is assumed that it will take some time for humans to rediscover the traditions and spells that were lost 1,000 years ago.
Golden dragons, particularly those outside of the former mazoku barrier, are generally practitioners of holy magic. Only a few dragons within the barrier, presumably those who lived before the Kōma War, are skilled in its practition. In order for humans to use holy magic again, they would need to translate the spells, now in dragon language, into literal human terms.
There are six known holy magic spells.
Some claim that the Di Flasher incantation is actually also a holy magic spell affecting the laser breaths of golden dragons. This seems to be possible, although there is no definite proof of this. Others believe that the teleportation technique which Filia Ul Copt used is also such a spell, because teleporting is not a natural ability of the dragons. In one of the issues of Blaster!, Hajime Kanzaka said that no teleportation or similar spells exist in the Slayers world, but later contradicted himself by introducing the black magic teleport spell Void, so the possibility of a holy magic teleport spell is certainly not out of the question.