The Light Domain created by the 40 Holy Paladins suppressed nearly 20 Legendary-level corporeal Vengeful Spirits. Its power was at least level-22 or level-23. The 40 Holy Light Knights were only a portion of the Holy Light Knights of the Brilliance Shrine's team. If all the Holy Light Knights were to be deployed, the Light Domain created would probably already reach the Legendary-level.
Most importantly, Donald thought about another aspect that was beyond the Sky Castle. In the Brilliance Shrine, the Holy Light Knights could be considered a regular force as there were Holy Light Knights stationed in almost every diocese. If thousands or tens of thousands of Holy Light Knights were to be sent out, what kind of Light Domain would be created?