I've often heard it said that in Ibn Taymiyyah's opinion, apostasy (without involving seditious acts) is a crime for which there is no earthly punishment. I haven't seen a reference for this though. In which work did Ibn Taymiyyah mention this (if at all)?
This is not true. It was a false claim made my Dr. Hisham Kamali and he has been proven wrong from Ibn Taymiya's own literature.
Ibn Taymiya made his views very clear in his book "As-Sarim al-Maslool 'ala Shatim ar-Rasool" 3/696
على خصوصها وكلاهما قد قام الدليل على وجوب قتل صاحبها والادلة الدالة على سقوط القتل بالتوبة لا تعم القسمين بل انما تدل على القسم الاول كما يظهر لمن ذلك تامل الادلة على قبول توبة المرتد فيبقى القسم الثاني وقد قام الدليل على وجوب قتل صاحبه ولم يات نص ولا اجماع بسقوط القتل عنه والقياس متعذر مع وجود الفرق الجلي فانقطع الالحاق والذي يحقق هذه الطريقة انه لم يات في كتاب ولا سنة ولا اجماع ان كل من ارتد باي قول او اي فعل كان فانه يسقط عنه القتل اذا تاب بعد القدرة عليه بل الكتاب والسنة والاجماع قد فرق بين انواع المرتدين كما سنذكره
شیخ الإسلام ابن تيمية رحمه الله - الصارم المسلول على شاتم الرسول، الجزء ٣، صفحة ٦٩٦
"...Apostasy is of two types: ordinary apostasy and extreme apostasy, for which execution is prescribed. In both cases there is evidence that it is obligatory to execute the apostate, but the evidence indicating that the sentence of death may be waived if the person repents does not apply to both types of apostasy. Instead, the evidence indicates that that is allowed only in the first case (ordinary apostasy) as will be clear to anyone who studies the evidence that speaks about accepting the repentance of the apostate. In the second type (extreme apostasy) the obligation to put the apostate to death still stands, and there is no text nor consensus to indicate that the death sentence may be waived. The two cases are quite different and there is no comparison between them. It does not say in the Quran or Sunna, or according to scholarly consensus, that everyone who apostatizes in word or deed may be spared the death sentence if he repents after he is a captured and tried. Rather the Quran and Sunna, and consensus, differentiate between the different kinds of apostates."
As you can see, he clearly believed in the death punishment.
It's interesting he mentions that the Qur'an differentiates between the different kinds of apostasy. In which case could he have been referring to Q4:90? If that's the case, wouldn't apostates who didn't fight muslims or apostates who joined people with whom we have a treaty of peace, be spared of the penalty? May 20, 2014 at 12:12
4:90 isn't referring to apostasy. But what I've read is the apostate is only punished if he/she makes it known to the public and separates from the jamaah. However, this doesn't exist today.– SayyidMay 21, 2014 at 11:22
Read from Q4:89. It clearly refers to people disbelieving after their belief. I'm not aware of any other verse in the Qur'an that differentiates between the different kinds of apostates. May 22, 2014 at 11:55
Read from 4:88, these verses are speaking about hypocrites.– SayyidMay 22, 2014 at 19:21
I know, but it doesn't really matter, it's the same principle. The hypocrites are openly Muslims and are officially recognized as such by the government. The subsequent verse (v89) speaks of when they openly show their disbelief i.e. apostatized. May 22, 2014 at 22:52