What 'projektir said - while religion is definitely changeable in theory in ways that race, gender, age, disability, etc. aren't, as a cultural phenomenon it's definitely heritable, and in particular, the parts that tend to stick are often the parts that aren't associated with ideology. If your social structure has been based around your church (or equivalent) since childhood, it's much easier to change your opinions on what things are moral or what directions a just government should take than to stop showing up. If your family goes to a certain worship service in the winter, you shouldn't face discrimination for going to that service too, and you shouldn't need to defend yourself by saying "I don't actually believe any of it" in public, for everyone including your family to hear.

(It's true that one way to solve this problem would be to highly regulate (or ban, but that's not quite necessary) religion and make sure that people's day-to-day interactions outside of worship are free from religion, with the aim of making it easier to change religions in practice. This would probably start with making it illegal for parents to take their children along with them to religious services or education. There are certain advantages there - but it would be such a huge change from existing free societies and such a huge infringement on how we currently treat religious liberty, that it's hard to say that this is how a free society should work.)

This is also why free societies with nondiscrimination-on-religion norms also tend to give partial but only partial deference to religious beliefs that conflict with secular laws. If it is important to you to have one day of the week off plus a few holy days, great, we can certainly protect your right to do that. If it is important to you that you not have to engage in war, that's fine but we'll still draft you into non-combat service. If it's important to you that you not execute certain laws as a government official or fill certain prescriptions as a pharmacist, that's also fine but you're going to need to find some career other than being an official a pharmacist.