Firstly, in any religion, there are always people who take the religion seriously and those who do not. Among them are those who apostate (leave the religion) and those who commit blasphemy (making offensive sacrilegious comments). Not everyone who commits such acts however is taken seriously and quite often those who make blasphemous comments are not given an opportunity to condemn their own religion openly and /or given a platform to insult their religion. Why is it then that when someone who comes from a Muslim background apostates and / or commits blasphemy, they are praised for their “bravery” (especially if they commit blasphemy), are taken seriously and given widespread media attention as if they are experts on Islam?
There are many examples of former Muslims who were granted much attention and high status such as Ayaan Hirsi Ali (former Dutch politician who is a critic of Islam who was honoured by Time Magazine as being one of the 100 most influential people in the world), Salman Rushdie (Indian author who wrote the “Satanic Verses” novel where he insulted Islam and was honoured with a Knighthood by the Queen of England), Majid Nawaz (former UK politician who founded a UK organisation that criticises Islam) and Maryam Namazie (former Iranian Muslim who is a broadcaster and a spokesperson for an organisation of ex Muslims). There are many others.
I am not a specialist on the subject but here are two links that you can read about that explain the Islamic rulings regarding those who apostate (from here) and those who commit blasphemy (from here). It is worth mentioning that while these rulings exist in Islam, there are similar beliefs in the other Abrahim faiths, Christianity and Judaism. Having said that, there is more to things than what we might first realise. As an example, Ubeidullah ibn Jahsh was a Muslim who was married to Ramlah bint Abu Sufyan (she later married the Prophet, Peace Be Upon Him, and was better known as “Umm Habibah”, may Allah be pleased with her). The two of them migrated together to Ethiopia as the early Muslims were persecuted by their own people. However, in Ethiopia, Ubeidullah ibn Jahsh was heavily influenced by Christianity and converted to Christianity. He made negative comments about Muslims when he saw them and was not killed but nonetheless died as a Christian.
Looking at things from the opposite perspective, if someone converts to Islam from another religion they still need to respect other religions and its followers as it is mentioned in the Quran:
وَلَا تَسُبُّواْ ٱلَّذِينَ يَدۡعُونَ مِن دُونِ ٱللَّهِ فَيَسُبُّواْ ٱللَّهَ عَدۡوَۢا بِغَيۡرِ عِلۡمٍ۬ۗ
And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge (Surah Al Anaam 6:108)
This also includes their parents if they are not Muslim:
وَإِن جَـٰهَدَاكَ عَلَىٰٓ أَن تُشۡرِكَ بِى مَا لَيۡسَ لَكَ بِهِۦ عِلۡمٌ۬ فَلَا تُطِعۡهُمَاۖ وَصَاحِبۡهُمَا فِى ٱلدُّنۡيَا مَعۡرُوفً۬اۖ وَٱتَّبِعۡ سَبِيلَ مَنۡ أَنَابَ إِلَىَّۚ
But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with appropriate kindness and follow the way of those who turn back to Me [in repentance]. (Surah Luqman 31:15)
Also, if someone converts to Islam from one of the Abrahamic Faiths, they need to respect all prophets and messengers including Musa (Moses) and Isa (Jesus), Peace Be Upon Them, as nobody can be a true Muslim without respecting them.
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