Saum is among the fundamental acts of Ibaadah. It has been ordained by Allah Ta'ala for the development of Taqwaa (piety) in the Mu'mineen. Fasting is extremely efficacious for the acquisition of Taqwaa. A Muslim cannot acquire Divine Proximity without Taqwaa. Without Taqwaa, the Muslim must necessarily drift far off the straight Path (Seeratul Mustaqeem) which leads to Allah Ta'ala and everlasting success in the Akhirah.
One who denies the fardhiyat (obligation) of Saum, no longer remains a Muslim and the one who does not fast during the month of Ramadhaan is a faasiq (an immoral and flagrant transgressor) of the highest order. Such a faasiq totally destroys his spirituality and morality and exposes his Iman to the gravest onslaughts of kufr.