The 25,000 members that left Norway's biggest church were actual participating members. No one devotes themselves to a religion, and publicly refers to themselves as indoctrinated, if they're not actively participating. Furthermore, the act of leaving denotes a level of participation. Those who left had participated long enough to realize that it was no longer the right fit for them.
Yes, the people who left Norway's biggest church were members and many of them participated at some level, at some point in time. If they weren't very active recently it merely points to their dissatisfaction, which ultimately led them to walk away from the church. Some will find other churches to participate in more fully.
No, the 25,000 members of Norway's largest church that left were not participating members. It does not make sense to believe that this many people who were dedicated to the church left. Norway does not have that many people or that many churches. Thus, many people may decided to be members of the same church.
If that many people left the church, they probably were not even that active in the first place. They just never had a public forum or system that allowed them to declare their status or attitude towards the church officially and in a way that can be counted such as these people were.