This topic has caused much confusion in Islamic circles primarily because those discussing it have completely misunderstood the issue due to a weak understanding of usul ul-fiqh. Instead of gaining the required knowledge they instead resort to slander and even takfir in some cases!
The question is not whether someone believes in a particular ahad hadith or not but the question is, who is a Muslim? What are those pillars of iman that someone must believe in to be called a Muslim, which if they reject they leave Islam?
Sheikh Ahmad Mahmoud sums up this discussion,
2 – In the pillars of the Islamic ‘Aqeedah, it is not enough to have zann (speculation) or least amount of zann, rather they must be qata’i (definite and decisive). It is not allowed to have taqleed (imitation) in it, otherwise the Muslims will end up taking superstition and follow those who practise deception.
3 – In the thoughts relating to the ‘Aqeedah (peripheral branches to the pillars) it is sufficient to have the least amount of zann, and taqleed is allowed in this matter, the same as it is in the Shar’ee rules.
Therefore in the pillars of the Islamic Aqeeda, i.e. what defines someone as Muslim these must be based on definite text with definite meaning. These are the Holy Qur’an and Hadith Mutawatir. In the branches of the Aqeeda then Hadith Ahad is fine to be used and believed in.
Sheikh Taqiuddin Al-Nabhani said:
“The solitary report (Khabr al-Ahad) is probable (zanni) evidence at best, so taking this as evidence for the ‘Aqeedah has been explicitly revoked in the Qur’an. The Shar’ai evidence and the reality of the ‘Aqeedah itself indicate that using the probable (Zanni) evidence for the ‘aqaa’id does not oblige the belief in what came in such daleel. Thus, the solitary report (Khabar al-Ahad) cannot be a proof for the ‘Aqa’aid.” 
Mohammad Hashim Kamali said:
“But Ahad may not, according to the majority of ulema, be relied upon as the basis of belief (aqidah). For matters of belief must be founded in certainty even if a conjecture (zann) may at time seem preferable. As the Qur’an tells us, ‘verily conjecture avails nothing against the truth’ (al-Najm, 53:28) Ahad, being conjectural, does not establish the truth.”
The dangers of basing the Islamic Aqeedah on conjecture are that sects (firqa) will arise who see each other as being deviant because one sect doesn’t accept the other’s belief. Today we see groups labeling Muslims as kafir and then slaughtering them even though unlawfully shedding Muslim blood is among the greatest sins.
وَمَن يَقتُل مُؤمِنًا مُتَعَمِّدًا فَجَزاؤُهُ جَهَنَّمُ خالِدًا فيها وَغَضِبَ اللَّهُ عَلَيهِ وَلَعَنَهُ وَأَعَدَّ لَهُ عَذابًا عَظيمًا
“As for anyone who kills a believer deliberately, his repayment is Hell, remaining in it for ever. Allah is angry with him and has cursed him, and has prepared for him a terrible punishment.”
A future Rightly Guided Caliphate will not adopt in the branches of Aqeedah. It will only adopt in the pillars of Aqeedah. This means the Islamic State will be a state for all Muslims regardless of their school of thought (mazhab). Differences of opinion will be resolved through the Islamic method that the classical scholars followed, and will not be resolved through bloodshed as we see today.