Allah, ta^ala, said in the Qur’an: 

وَلَئِن سَأَلْتَهُمْ لَيَقُولُنَّ إِنَّمَا كُنَّا نَخُوضُ وَنَلْعَبُ قُلْ أَبِاللّهِ وَآيَاتِهِ وَرَسُولِهِ كُنتُمْ تَسْتَهْزِؤُونَ 

لاَ تَعْتَذِرُواْ قَدْ كَفَرْتُم 

Qul abillahi wa ayatihi wa rasulihi kuntum tastahzi’un. La ta^tadhiru qad kafartum ba^da imanikum. 

Ayahs 65-66 of Suratut-Tawbah mean: {O Muhammad, say: Do you mock Allah, His verses, and His Messenger? Do not offer excuses. Now you are judged as blasphemers, after you were judged as Muslims.} 

 Apostasy (Riddah) is leaving Islam. It is the most abhorrent type of blasphemy. Hence, it is obligatory on every Muslim to preserve his Islam and protect it from apostasy (riddah), which invalidates his Islam. May Allah protect us from it.