Some of the things are legitimate rules, like the fact that priests need to be celibate. Other things are immutable principles that cannot be changed, like the

Some of the things are legitimate rules, like the fact that priests need to be celibate. Other things are immutable principles that cannot be changed, like the fact that a Baptism requires the use of water (and not e.g. soda or orange juice) for it to be valid. The rules can be (and have been) changed, because the people in charge of them were given authority to institute and dissolve their own rules. The laws (in the sense of "laws of nature" rather than "laws of government") cannot be changed.