"This is where the border of the Western Continent lies," Cardinal Treniel informed Eren. "Beyond this river is the Northern Continent."
The Cardinal knew that he was going to be weaker the moment he left the borders of the Western Continent as the blessing of the goddess couldn't reach the Northern Continent.
Still, he didn't tell Eren about it. He didn't want Eren to know that he was weaker, not wanting him to think about returning to bring more people to be safer.
He didn't realize that Eren already knew about it. In fact, it was the reason he was accompanying the Cardinal in the first place.
"Do we need to take any special measures before entering the Northern Continent?" Eren asked the Cardinal. "Can they detect us the moment we cross the border?"