Many Mullahs and Imams preach the death penalty for apostates who abandon Islam after having been Muslims.
Is this penalty authorised by Allah in the Quran? And if not, how can we answer such false preaching?
God gives us in the Quran all the proofs to expose those who preach what is contrary to the law of God in the Quran.
In Surah 4 Verse 137
إِنَّ الَّذِينَ آمَنُوا ثُمَّ كَفَرُوا ثُمَّ آمَنُوا ثُمَّ كَفَرُوا ثُمَّ ازْدَادُوا كُفْرًا لَّمْ يَكُنِ اللَّهُ لِيَغْفِرَ لَهُمْ وَلَا لِيَهْدِيَهُمْ سَبِيلً
Inna allatheena amanoo thumma kafaroo thumma amanoo thumma kafaroo thumma izdadoo kufran lam yakuni Allahu liyaghfira lahum wala liyahdiyahum sabeelan
Indeed, those who believed, then disbelieved, then (again) believed, then disbelieved, then increased (in) disbelief - not will Allah forgive [for] them and not will guide them (to) a (right) way.
In 4:137, God speaks of those who believed, then disbelieved, then believed, then disbelieved.
The question here is: if a Muslim is put to death as soon as he abandons the faith, then how would it be possible for him to believe again, and then disbelieve again?
Surely, he would be dead before this sequence happens!
As a result, to apply the death penalty on anyone who reverts would render God's words in 4:137 impossible to happen, and thus invalid!
Rather, God's words in 4:137 prove that there is no penalty of death, nor any other penalty, for those who choose to disbelieve.
Further confirmation is given in 18:29 where God gives all humans the free choice to believe or disbelieve without being penalised or killed:
Surah 18 Verse 29
وَقُلِ الْحَقُّ مِن رَّبِّكُمْ فَمَن شَاءَ فَلْيُؤْمِن وَمَن شَاءَ فَلْيَكْفُرْ إِنَّا أَعْتَدْنَا لِلظَّالِمِينَ نَارًا أَحَاطَ بِهِمْ سُرَادِقُهَا وَإِن يَسْتَغِيثُوا يُغَاثُوا بِمَاءٍ كَالْمُهْلِ يَشْوِي الْوُجُوهَ بِئْسَ الشَّرَابُ وَسَاءَتْ مُرْتَفَقًا
Waquli alhaqqu min rabbikum faman shaa falyu/min waman shaa falyakfur inna aAAtadna lilththalimeena naran ahata bihim suradiquha wa-in yastagheethoo yughathoo bima-in kaalmuhli yashwee alwujooha bi/sa alshsharabu wasaat murtafaqan
And say, "The truth (is) from your Lord, so whoever wills - let him believe and whoever wills - let him believe." Indeed, We have prepared for the wrongdoers a Fire will surround them its walls. And if they call for relief, they will be relieved with water like molten brass, (which) scalds the faces. Wretched (is) the drink, and evil (is) the resting place.
To conclude, there are no penalties of any kind mentioned in the Quran for apostasy. The penalty of such an act lies with God alone.
Peace and Respect!