the chofetz chaim in ahavas chesed(end of perek 7) quotes a midrash koheles that says anyone who is a kofer towards gemillas chesed is a kofer b'ikar. he explains that since Hashem is the mekor of goodness and chesed and the whole world was created to benefit the creations(us) like it says in the passuk " ....l'tov lanu kol hayamim" then it must be that anyone who denies all areas of middas chessed is a kofer b'ikar.