In the Name of Allah, most Compassionate, most Merciful

Takfir means to call a person as disbeliever who has earlier accepted Islam.  This article only points out to the fact that I am not making Takfir on any person who says the shahadah because it was an order by prophet (S).  Even if a Muslim does an act which takes him out of fold of Islam and should be killed for or to be moved away from, we do not find Sahabas making Takfir on them.  

Not making Takfir in no way proves that a Muslim can never be in hellfire forever or should not be killed, but there are many things if a Muslim does and die without correcting and repenting then he can be in hellfire forever.  For example, if a Muslim dismantles the Shariah or does Usury, will be in hellfire forever unless he repents and corrects him/herself.   Many hadith and ayahs of Quran proves it if one looks for it.  Also it is generally summed up in this ayah of Quran...

100. Whoever turns away from it (this Quran i.e. does not  act on its orders), verily, they will bear a heavy burden on the Day of Resurrection,

101. Forever in it (in that state), and evil indeed will it be that load for them on the Day of Resurrection.

What is our action towards those Muslims who do open kufr and shirk? There are two options after enjoining good, either separate ourselves or if Sharia is implemented then bring him to justice.  Rasool (S) said that it is only the Pious Mo'min who remains at liberty from others and only then is his life sacred.  It excludes any Muslim who commits acts of kufr or shirk and these things take him out of the sphere of being a Mo'min.  And thus what Abu Bakr (R) did to the refuters of zakat as justified.

Sahih Al-Bukhari Hadith 9.2 Narrated by Ibn Umar
Allah's Apostle said, "A Mo'min remains at liberty regarding his religion unless he kills somebody unlawfully."  

If there is no Shariah on the land then the only other option we got is what Rasool (S) instructed us... to leave all the sects until death overtakes you.  As we can see today that Muslims have divided into many sects and there is no Shariah anywhere.  The only obligation we have in this case is to leave all the sects of Muslims and isolate oneself.  This in no way means to live among disbelievers or Sectarian Muslims themselves.

Sahih Al-Bukhari Hadith 4.803 Narrated by Hudhaifa bin Al Yaman
"...I asked, "What do you order me to do if such a thing should take place in my life?" He said, "Adhere to the group of Muslims and their Chief." I asked, "If there is no group Muslims and their chief (what shall I do)?" He said, "Keep away from all those different sects, even if you had to bite the root of a tree, till you meet Allah while you are still in that state."

What does it mean by no group (jama'ah) of Muslims and their chief?  It means that the Muslims have not divided into sects and all the Muslims belong to the same aqeedah and their leader is one (i.e. Umayyad and Abbassid caliphate).  Sects here mean that Muslims will have different Aqeedahs and will only lead to Hellfire and there would not be any Shariah.  Fighting them or correcting them will only increase in trouble for the believer so therefore it is best to save one's deen and move away where the deen would be safe.