A faith that requires adherents to just have the correct beliefs without placing any physical obligations on them should theoretically be the easiest to follow. So by extension that faith would have the most members and would constantly keep attracting new members.

A faith that requires adherents to not only possess the correct beliefs but also physically practice it by committing various actions (recitations, multiple daily prayers, fasting, parting with wealth, memorizing scriptures, abstaining etc) should theoretically be the dullest and most difficult to follow. So by extension it would have the least members and would constantly keep losing members.

But the exact opposite is true -- The "easy" faith is the one that's dwindling. While the "difficult" faith is preserved from generation to generation.

Why?

It's the same reason why the one who eats right and exercises daily is in better shape than the one who merely believes in the importance of right diet and exercise but doesn't perform actions supporting his beliefs.

An easy beliefs based faith leaves you free enough to physically participate in non-faith activities. You are not obliged to pray multiple times a day so you think you're free to watch multiple episides on Netflix. You're not obliged to memorize scripture so you read a comic instead. So the faith gets pushed to the back of the mind. Its just there and you're not doing anything to grow it. Instead you're growing your interest in Hollywood, video games, Netflix etc.

Eventually, the faith becomes too weak to influence your life. So you start living a lifestyle that resembles that of the average non believer. There's also the risk that you start exposing yourself to materials that plant the seeds of doubt in your mind so you start abandoning your faith.

In contrast, a difficult beliefs + actions based faith keeps you occupied with various actions of worship. You are not as free to participate in non-religious activity. Either because its forbidden. Or because you have some other religious obligation to fulfill. If it's time for prayer you tend not to press play on the latest Netflix episode. If its time to memorize a passage you tend not to start a new game of Call of Duty. There's a constant tug of war between the "faith" part of your brain and the part that just wants to have fun.

So by exercising it daily your faith becomes the CENTER of your daily life, not just a thing you believe. The more you physically practice it, the stronger it becomes. And the stronger it becomes, the more it influences your thinking and your daily life. And the more that occurs, the more likely you are to stay on the faith, resist temptation and avoid vices.