By only refusing to acknowledge the Imamate...
By only refusing to acknowledge the Imamate, a person cannot be excluded from the Islamic fold. Of course, on the Day of Judgement, and in the stages of nearness to God and "karamat" (miraculous signs), the Shi'a faith will reveal its excellence over the Islam of the majority. All Muslims are equal in the world and are brothers, but in the next world there will certainly be a difference of ranks. People will be accorded positions according to their performance and intention.
The final decision is in God's hands in these matters, and it is therefore better for us not to involve ourselves. We have explained that that which distinguishes the Shi'a from the Sunni Muslims is their belief in the Imamate of the Twelve Imams, and it is for this reason that this sect is called "Imamiyah". It should be noted that all Shi'as are not Imamiyah, because the word Shi'a applies to the Zaydiyah, the Isma'iliyah, the Waqifiyah and the Fathiyah as well.
These are sects which are counted as Muslim. But a more careful study of other sects will show that there are many which are totally excluded from Islam, but which, nevertheless, are sometimes still called Shi'a as, for instance, the Khitabiyah, who are an example of the hundred or more sects which cannot be regarded as Muslim. At the present time, however, the word of Shi'a specifically refers to the Imamiyah sect; which is the biggest body of Muslims in the world of Islam after the Sunnis.
In Islamic learning, faith in the twelve imams is not anything new; it is referred to in all reliable and authentic books by Muslim authors. The Imams Muslim and al-Bukhari narrated traditions concerning the twelve Imams in their Sahihs in different ways.
A few of them are given here: Jabir ibn Samrah says, "One day when I went to the Holy Prophet (s.a.w.) along with my father, the Holy Prophet (s.a.w.) said: 'The universe shall not come to an end until all the twelve caliphs have appeared.' After this the Prophet (s.a.w.) said something silently, which I could not hear. I asked my father what the Prophet of Islam (s.a.w.) had said.
He said: "The Holy Prophet (s.a.w.) says that all of them shall be from the Quraysh." Another tradition says: "So long as there are the twelve authorities, the Muslims will continue to exist." Again: "So long as there are the twelve caliphs, the grandeur and majesty of Islam shall live on. God knows who these twelve caliphs are.