Rasul Allah (sal Allahu alaihi wa sallam) said: “Whoever breaks a fast of Ramadan without an excuse or dispensation could not requite it by fasting a lifetime, were he to do so.” [Tirmidhi]
Allah (subhana wa ta’ala) has excused the ill, the traveler, the pregnant and nursing women from keeping the fasts of Ramadan. Some are required to make the fasts up later while others may pay Kaffara, feeding the needy, instead. If someone does not keep the fasts of Ramadan without having an excuse from the Shariah, their sin is enormous. Making up those missed fasts of Ramadan, while obligatory, cannot remove the sin. One must also make a sincere repentance to Allah (subhana wa ta’ala) in order to be forgiven.
Imam Adh-Dhahabi says: “According to the established believers, anyone who leaves the fast of Ramadhan without being sick is worse than a fornicator or an alcoholic. In fact, they doubt his Islam and they suspect that he might be a zandiqah (one who claims to be Muslim in order to destroy it from within) and one of those who destroy Islam.” [quoted in Fiqh us-Sunnah by As-Sayyid Sabiq]