Apostacy is a big deal and InshaAllah if you sincerely repent unto Allah(swt), Allah(swt) will forgive you. Remember that Allah(swt) is always willing to forgive us and He does give us so many ways to do good.
Merriam-Webster sin 1 a: an offense against religious or moral law b: an action that is or is felt to be highly reprehensible <it's a sin to waste food> c: an often serious shortcoming 2 a: transgression of the law of God b:a vitiated state of human nature in which the self is estranged from God
Even atheists sin despite denying the very existence of God. Disbelief in the One God is in fact a major sin.