«فَفِي رَحْمَتِ اللَّهِ هُمْ فِيهَا خَالِدُونَ»
They will remain forever in Allah's mercy—that is, in Paradise.
Mercy is manifested in Paradise. The word “mercy” here directs attention to the place in which that mercy is enjoyed. The containing place is meant through something located or realized in it.
This is haliyya. In mahalliyya the container is named for its occupants; in haliyya the contained state or thing is named for its place.