Praise be to Allah سبحانه وتعالى, the Lord of the worlds. May Allah The Exalted raise the rank of our master Muhammad صلى الله عليه وسلّم, his Al and Companions, and protect his nation from that which he fears for it.

Fasting the month of Ramadan is an obligatory and great worship. It is among the best acts of obedience. In Fasting there is a great reward. Imam al-Bukhariyy related the Qudsiyy hadith that the Prophet صلى الله عليه وسلّم said that Allah سبحانه وتعالى said:

{كلّ حسنة بعشر أمثالها إلى سبعمائة ضعف إلا الصيام فإنه لي وأنا أجزي به}

which means: <<The reward of every good deed is multiplied from ten up to seven- hundred times. However, the reward of Fasting, done in sincerity, will be multiplied by as many times as Allah سبحانه وتعالى wills.>>, i.e., more than 700 times, One of the doors of Paradise, named ar-Ray‬‎yan, is specified for those Muslims who used to fast in this world. On the Day of Judgment, it will be opened, and those who used to fast in obedience to Allah تعالى will be called to enter through that door. After they enter, ar-Rayyan will be closed, and no one else will enter through it.
The month of Ramadan is the ninth month of the lunar year. It is the best month of the year, as the night of al-Qadr in Ramadan is the best night of the year. Fasting became an obligation on the Muslims in the second year after the Prophetic migration. Prophet Muhammad صلى الله عليه وسلّم fasted Ramadan for nine years, after which he died.

The obligation on the believers of fasting Ramadan is known from the Qur'an and the Hadith. It was explicitly stated in Verse 183 of Surat al-Baqarah:

{يا أيها الذين ءامنوا كتب عليكم الصيام كما كتب على الذين من قبلكم لعلكم تتقون}

which means: [O believers, Fasting is an obligation upon you as it was ordained upon the (Muslim nations) before you to help you become pious.] Fasting was an obligation upon the nations before the nation of Prophet Muhammad صلى الله عليه وسلّم. However, fasting the month of Ramadan in particular is a specification for the nation of Prophet Muhammad صلى الله عليه وسلّم only.

Moreover, Prophet Muhammad صلى الله عليه وسلّم named Fasting the month of Ramadan as one of the five most important matters of Islam in the famous hadith, known as " Hadith Jibril", as related by al-Bukhariyy and Muslim. Angel Jibril came to the Prophet صلى الله عليه وسلّم in the shape of a man with white clothes and asked the Prophet peace be upon him to inform him about Islam. In response, the Prophet صلى الله عليه وسلّم said:

{ الإسلامُ أنْ تشْهدَ أنْ لا إلـهَ إلا الله وأنَّ مُحَمَّدًا رسولُ الله ، وتُقيمَ الصلاةَ ، وتُؤْتِيَ الزَكاةَ ، وتَصُومَ رَمضانَ ، وتَحُجَّ البيتَ إنِ اسْتطَعْتَ إلَيهِ سَبيلا }

which means: <<Islam is bearing witness that no one is God except Allah and Muhammad صلى الله عليه وسلّم is the Messenger of Allah, performing Prayer, paying Zakah, Fasting the month of Ramadan, and performing Pilgrimage if you are able.>>
Consequently, for the one who has learned of its obligation, renouncing the obligation of Fasting the month of Ramadan is blasphemy, because it entails belying Allah and the Prophet صلى الله عليه وسلّم. However, the one who believes Fasting is an obligation, yet does not perform it (without a valid excuse) does not blaspheme, although his neglecting to fast is an enormous sin.