Question Summary:
Explain/clarify the wisdom or reason why an action can take one out of Islam even if done purely as a joke?

Question Detail:

 
I am an intermediate student of knowledge in the Arab world.
The scholars go to great lengths to mention that sins of the tongue and body (like fornication, and abandoning salat) do not take one out of Islam, but they still say that certain actions do take one out of the fold of Islam (like throwing the Quran in the bin, and making statements of disbelief even if only jokingly).
When I looked through the books of fiqh I found in the chapters of ridda (apostasy) that certain strong statements and actions taken one out of Islam (Dusuqi (Maliki) Ibn Nujaym (Hanafi), Ibn Hajar (Shafii). Ibn Abidin (Rahimahum Allah jamee'an) mentions:
والحاصل أن من تكلم بكلمة الكفر هازلا أو لاعبا كفر عند الكل ولا اعتبار باعتقاده
(Whoever speaks words of kufr criticisingly or jokingly (in jest) has done kufr according to all (scholars) regardless of his belief (in his heart)) 
 
However Imam Tahawi says:
Nobody leaves Islam except through (juhood - rejection) of what brought him into it.
 
My question is:
Part one:
Is it the position of all the scholars of all the madhabs that certain actions of the tongue and body can take one out of Islam even if the heart is Muslim or were the earlier scholars like Tahawi of the opinion that only actions coupled with beliefs constitute kufr?
Part two:
If it is the position of all the scholars, can you please explain/clarify the wisdom or reason why an action can take one out of Islam even if done purely as a joke - for example someone jokingly pretends to be a Christian etc.) when one's heart is still Muslim?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh. 
In principle if a person commits any sins, whatever major or minor besides kufr and shirk, the sins alone do not take a person out of the fold of Islam[1].
However, if one commits mockery of Deen (Istihza`), or humiliation on Deen (Ihana) or looks down upon on orders of Deen (Istikhfaf), then the person exits the fold of Islam[2]. The reason for this is each one of the three is against Tasdeequl Qalb (Testifying of Iman). If one has real tasdeeq, why would he make Istihza`, Ihana and Istikhfaf? In essence, such  a person does not have a real tasdeeq of Iman, therefore he exists the fold of Islam. It is not due to any amal.
 
Abu Ali ad-Dagestani
Moscow, Russia
Checked and Approved by,
Mufti Ebrahim Desai