There will be a religion in most of them. Islam.
The Christian religion is not what will become extinct. It is the traditional populations of these countries that is becoming extinct.
Demography is destiny. The new arrivals who are adherents of Islam have much greater fucundity. They will displace the current population. It is nearly inevitable.
It is probably the case that the time devoted to religion is reduced, as it competes with other activities. This will by many be interpreted as the end of religion, but I do not think so. We will still subscribe to Judeo-Christian values. Islam is different, it is a political religion, with a strange mind set that is foreign to Western values and has to be opposed.