Firstly: The Ruling if Fasting Ramadan
Fasting Ramadan is an obligation and one of the five pillars of Islam. Consensus has been quoted over this matter by: Ibn Qudamah, al-Nawawi, and Ibn Taymiyyah.
Secondly: The Ruling of the One who Does Not Fast Denying its Obligation
Whoever does not fast Ramadan out of denial of its obligation then he has disbelieved.  Unless he is new to Islam, or was raised away from Muslims, such that it is conceivable that he was unaware of its obligation. In this case, he does not leave the fold of Islam. Consensus has been quoted on this by al-Kasani.
Thirdly: The Ruling of the One who Does Not Fast Ramadan Deliberately out of Laziness
Whoever does not fast a single day of Ramadan deliberately out of laziness, then he has committed a major sin, and he must make up that day. This is by agreement of the four schools of jurisprudence.