From basic evolutionary biology, there is no universall "fitness", only situational fitness.
In the middle ages, in a world without contraception, with extreme pressures to marriage, without pornography or any other non-intercourse sexual releases, being religious or not matters little in fertility rate.
Although in frankness, it still did, that's how rome became christian in the first place, Christians married later (so had higher chances of survival), and treasured their baby girls instead of exposing them to death, small fertility advantages added up expontentially over time.
In the 21st age, the environment has shifted so much against irreligious/low commitment people in terms of reproductive fitness, due to those above factors heavily selecting against them.
Literally only religious people, no matter what part of the world you are looking at, are able to maintain above replacement fertility. Irreligious people' population decline, in the long run, at least 25% every generation, will be far faster than they can "convert" religious people.
Look at the amish and mormon fertility rates, read decades of research by Eric Kaufmann, and the answer is rather self evident