Ultimately it is meaningless since all religion is on the decline.
Islam is rising. there are actually more muslims in the world today than there were 100 years ago
also there is a surge in neo paganism
There is a rise in absolute numbers of all religions, as a consequence of population growth. But because of the internet and the spread of technology there is an undeniable decline in religious fervor and religious influence over governing institutions. That is decline.
I'm sure with time we will see less absolute numbers. With Islam there could already be a huge falling out from the faith but we would never know since apostates are executed in the major Islamic countries.
well in the case of paganism it is honest interest and 99% of pagans are not born into it but choose to become it. it might not be a world overtaking group, but paganism is a rising religion I think that could someday be a bigger force in the world