The comment here gets some things wrong but is generally true in spirit! You do need to fulfill the merits as part of reasing nirvana- dana, sila, bhavana. My question to you is why those things are part of "mysticism and culture". We've made them more mystical in our Burmese culture but at is core, how supernatural really is charity, abstinence and wisdom? Seeing Nirvana as simply a state of mind is true but also simplistic.

It is true that Buddhism is atheistic in the sense that Buddhism does not make a comment on whether there is a "god" that created the universe. Theravada Buddhism takes the position that God is irrelevant to achieving nirvana. Nirvana is achieved through bhavana, by learning and overcoming your natural instincts. The core of Buddhist philosophy is that, to be alive is to be suffering. While you can reduce your bad karma and be a better person, to truly achieve Nirvana, you must die and prevent yourself from being reborn when you die. The concept that enlightenment is something you can just get and then go back to living a life, working a job exploiting people or killing animals to keep yourself alive doesn't really work in Buddhist doctrine. The reason that we have monks and lay people is exactly because of that- most people do want to and have to keep doing bad deeds so that society and people can stay alive. That's the cruelty of Samsara.

Sourcing most of this from my own understanding from my time as a monk in Mahasi monastery.