The Holy Church of Light... It was said to be one of the strongest powers in the World especially because of the Holy Priest of Light who was claimed to be the strongest of all the Holy Priests according to the people of the world.
Under normal situations, it was almost impossible to infiltrate the Holy Church of Light, but these weren't the normal situations. Half the Church of Light was out of the city, possibly in some war so their strength was already half of their peak.
Moreover, whatever half was left, he had a feeling that those numbers were going to dwindle even more if the Academy of Elements called for help.
Before this month, he hadn't even thought of infiltrating the Church of Light because there was a possibility of the Holy Priest being there. The Head Priests of Light were also more sensitive to the Darker Elements so there was a good chance that his Darkness could've been sensed if he even tried entering.