If not, you are in rebellion and unorthodox, and your immortal soul is in danger.
If you are not a devout, orthodox, obedient Catholic, you cannot commit the big sins. But, if you depart from Church teaching in ANY way, then you will be open to great temptation and mortal sin.
Think about this.
Here is the Merriam-Webster definition:
her·e·synoun \ˈher-ə-sē, ˈhe-rə-\
Full Definition of HERESY
a : adherence to a religious opinion contrary to church dogmab : denial of a revealed truth by a baptized member of the Roman Catholic Churchc : an opinion or doctrine contrary to church dogma
a : dissent or deviation from a dominant theory, opinion, or practiceb : an opinion, doctrine, or practice contrary to the truth or to generally accepted beliefs or standards