عَنِ ابْنِ مُسْكَانَ عَنْ اَبِي عَبْدِ اللهِؑ قَالَ مَنْ اَنْكَرَ وَاحِداً مِنَ الْاَحْيَاءِ فَقَدْ اَنْكَرَ الْاَمْوَاتَ
Abdullah Ibn Muskaan narrates from Imam Abu Abdillah al-Sadeq (peace be upon him), “Whoever denies anyone of the living ones then indeed he has denied the dead ones.”
Note:
Thus, anyone who denies a living Imam has denied the past Imams (peace be upon them). When Imam Reza (peace be upon him) was brought from Madinah to Marv by Mamoon al-Abbasi the accursed, among the incidents that occurred was that Mamoon invited the scholars of different religions from different parts of the world so that they may debate with Imam (peace be upon him). He wanted them to defeat Imam (peace be upon him) so that he could prove that Imam (peace be upon him) was not a divine appointee and an ordinary person.
In one of these debates, a Christian monk asked Imam (peace be upon him),
قَالَ الْجَاثَلِيقُ مَا تَقُولُ فِي نُبُوَّةِ عِيسَى وَ كِتَابِهِ هَلْ تُنْكِرُ مِنْهُمَا شَيْئاً قَالَ الرِّضَاؑ اَنَا مُقِرٌّ بِنُبُوَّةِ عِيسَى وَ كِتَابِهِ وَ مَا بَشَّرَ بِهِ أُمَّتَهُ وَ اَقَرَّتْ بِهِ الْحَوَارِيُّونَ وَ كَافِرٌ بِنُبُوَّةِ كُلِّ عِيسًى لَمْ يُقِرَّ بِنُبُوَّةِ مُحَمَّدٍ ﷺ وَ بِكِتَابِهِ وَ لَمْ يُبَشِّرْ بِهِ أُمَّتَه
(Behaar al-Anwaar, vol. 10, p. 301)
‘Do you believe in Eesa (peace be upon him)?’ The purpose of his question was that if Imam (peace be upon him) replies positively, then he would say that ok, you believe in Eesa but we don’t believe in Muhammad (peace be upon him and his holy progeny). So, there is consensus about Eesa but not for Muhammad. Imam (peace be upon him) replied, ‘I believe in Eesa who gave glad tidings about Muhammad (peace be upon him and his holy progeny).’
That is, I believe in the religion of Eesa completely without any deficiency i.e. the one that includes the prophecy about Holy Prophet (peace be upon him and his holy progeny).
Similarly, one who believes in Ameerul Momineen Ali Ibn Abi Talib (peace be upon him) has to believe in Imam Reza (peace be upon him) and all Imams (peace be upon them). One cannot say that I believe in all Imams (peace be upon him) except Imam Muntazar (a.t.f.s.).
Obviously, in the above tradition, the usage of the word living and dead is metaphorical because Imams (peace be upon them) even in their death are living.
References:
1. Behaar al-Anwaar, vol. 23, p. 95, H. 1
2. al-Kaafi, vol. 1, p. 373, H. 8
3. Kamaal al-Deen, vol. 1, p. 14 and vol. 2, p. 410, H. 1