Lith and Dennis were creating a ruckus in the dark church. But they weren't too loud and knew their limits and thus neither Emilia nor the priest stopped them from being near the symbol and saying those things.
The priest was present in the church from the start when the four entered and since the church was visited by people daily, he didn't bother them and these four didn't bother the priest either.
The priest did hear the conversation of the three but some of the things he didn't understand and what he did make out was that three young teens were just pulling pranks and teasing each other. It felt normal for teens of their age to do such things and as long as they didn't cross any boundaries, he was fine with them doing whatever they wanted.
"Oi, oi, what are you two doing! Stop it!" Ralph ran towards them and pulled their collars and tried to take them away from the cross.