if your born into a catholic family. does that mean you will practice the faith forever?\
Usually yes.
Sometimes you'll change faith into another.
On rare occassions you'll denounce all the gods.
All the same one born into atheist family may become religious.
90% of population in my country was baptised Catholic; 80% of the population is considered to be practising Catholics.
All the rest is usually different faiths, while amount of atheists is minimal. Much depends on the influence a Church takes and the mentality of the population.
Let's just remind ourselves the mentality of Yliakum's people is that of the middle ages, not modern times. Let's remind ourselves that the population has wide access to magic, which opens their eyes to the supernatural. Let's remind ourselves that the temple of Talad tends to spawn strange phenomena, considered to be miracles and available for anyone to see.
Waylander's 75% still stands as a valid estimation.