What is the ruling on one who abandons the Sunnah, justifying it by a lack of conviction and by claiming that the religion is easy?

Neglecting Sunnah prayers is not a sin in the same way as neglecting obligatory prayers, but consistently abandoning them indicates a deficiency in one's religious practice. However, if they are abandoned out of disdain or belittlement of their importance, then this is not permissible and is considered an act of defiance (fisq), due to the Prophet's (peace be upon him) saying: "Whoever turns away from my Sunnah is not of me." As for denying the Sunnah and its authority, that is an disbelief.