Islamic beliefs are like a connected chain, such that if one part is missing, then the whole chain will be useless and have no benefit. In other words, believing in Allah has its entailments; belief in these entailments is what makes the difference and brings about Allah’s satisfaction. If you examine the story of Satan being expelled from heaven, you will find that he asked Allah for the chance to live until the Day of Judgment to deviate man.  Therefore, it is understood that Satan believed in Allah and the Day of Judgment but Allah still names him a Kāfir.  So just believing in Allah is not enough for one’s belief to be accepted, but one must also stay firm in regard to the results and implications of this belief. One of the results of one’s belief in Allah is belief in His messengers meaning the prophets, in such a way that denying them would be like denying Allah. Also, believing in the Imams, meaning the prophets’ successors, is the inevitable result and reflection of one’s belief in the prophet, because the Imams are the prophet’s successors that were appointed by himself and denying them is like denying the prophet himself. Therefore, one cannot believe in the prophet and not believe in the imams. It is the same between each one of the imams, meaning that denying one of them is like denying all of them, for the reason that proves the Imamate of each one of them is the same and applies to all of them; all of them are the successors of the prophet and each Imam has been approved of by the previous one and has introduced the next Imam. Therefore, denying one of them leads to the denial of the previous one and because all of them were appointed by the prophet, denying them is denial of the prophet.
Finally, we will mention a few ahadith in this regard: Imam Sadiq narrates from his great ancestors, meaning the previous Imams, that they narrated from the prophet that he said: “My successors are twelve individuals, the first is Ali bin Abi Talib and the last is the ‘Ghā’im’, they are my successors, vicegerents, the leaders after me and my Hujjats (reasons) over my Ummah, whoever bears witness to their imamate is a Mu’min (Believer), and whoever denies their Imamate is a kāfir (disbeliever).” 
It is narrated in another hadith by Imam Sadiq that: “One who does not follow our words has rejected Allah’s words, and one who rejects Allah’s words is a polytheist”. 
Also, it is mentioned in another hadith by Imam Kadhim that: “One who has hate for us resents the prophet of Allah and he actually has hate for Allah, and one who has hate for Allah deserves the hellfire and will not receive help from anyone.”   “قالَ رَبِّ فَاَنظِرنِی ِالَی َیوِم یُبعَثُون …” Hijr:36; Sād:79. Baqarah:34; Sād:74.  “عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ عَنْ آبَائِهِ عَنِ النَّبِیِّ ص قَالَ الْأَئِمَّةُ بَعْدِی اثْنَا عَشَرَ أَوَّلُهُمْ عَلِیُّ بْنُ أَبِی طَالِبٍ وَ آخِرُهُمُ الْقَائِمُ فَهُم خُلَفَائِی و أَوصِیَائِی و أَولِیَائِی وَ حُجَجُ اللَّه عَلَی اُمّتِی بَعدِی الْمُقِرُّ بِهِمْ مُؤْمِنٌ وَ الْمُنْکِرُ لَهُمْ کَافِرٌ”، Sheikh Saduq, Man lā Yahduruhū al-Faqīh , vol. 4, book of wasiyyah (i.e., making a will), the chapter on the will of Adam (as), pg. 180,…hadith 5406, Jame’eye Mudarresin Publications, Qom, 1413 AH. Of course, the term ‘kufr here is to be taken in its literal sense, which is to hide and conceal the truth, and not in its common usage.  “الرَّادُّ عَلَینَا الرَّادّ عَلَى الله وَ هُوَ عَلَى حَدِّ الشَّرکِ بِاللهِ…” Kuleini, vol. 1, chapter of difference in hadith, pg. 68, Dār al-Kutub al-Islamiyyah Publication, Tehran, 1365 (solar calendar).  “ألا یا عبد الرحمن من أبغضنا فقد أبغض محمدا و من أبغض محمدا فقد أبغض الله جل و علا، و من أبغض الله جل و علا کان حقا على الله أن یصلیه النار و ماله من نصیر” Majlisi, Muhammad Baqir, Bihār al-Anwār, vol. 97, pg. 122, Al-Wafā’ Institute, Lebanon, 1404 AH.