Muhammad Baqer Majlese said:
اعلم أن إطلاق لفظ الشرك والكفر على من لم يعتقد إمامة أمير – المؤمنين والائمة من ولده عليهم السلام وفضل عليهم غيرهم يدل على أنهم كفار مخلدون في النار
“U should know that usage of words shirk and kufr regarding people who don’t believe in the imamate of amiralmominin (a.s) and imams from his descendants, is a proof that such people would remain in the hell forever”. Source: “Bihar al anwar” 23/390

Shias narrated that prophet (sallalahu alayhi wa salam) said:
من جحد عليا إمامته من بعدي فانما جحد نبوتي ومن جحد نبوتي فقد جحد ربوبيته
“Who would reject imamate of Ale after me, he rejected my prophecy, and whoever rejected my prophecy, he rejected rububiyatihi”

And they reported from Sadiq:
المنكر لآخرنا كالمنكر لاولنا
“Rejecting the last one from us, is like rejecting the first one from us”.
Source: “Bihar” 27/61.

So even if we would accept all of them, and reject the last one we are still kuffar in their view.

And they reported from prophet (sallalahu alayhi wa salam)
الائمة من بعدي اثنا عشر أولهم أمير المؤمنين علي بن أبي طالب عليه السلام وآخرهم القائم ( 1 ) طاعتهم طاعتي ومعصيتهم معصيتي ، من أنكر واحدا منهم فقد أنكرني
“There are 12 imams after me, first of them is Ale ibn Abi Taleb, and last one is Qayum, obedience to them is obedience to me, insubordination of them is insubordination of me. AND WHOEVER WOULD REJECT ONE OF THEM, HE REJECTED ME”. Source: “Bihar” 27/61.

And in “Bihar” we can also read:
ومن أنكرهم أو شك فيهم أو أنكر أحدهم أو شك فيه أو تولى أعداءهم أو أحد أعدائهم فهو ضال هالك بل كافر
“Whoever reject them (imams) or doubt in them, or reject one of them, or doubt in one of them, or (whoever) would be a friend to their enemy, or (even) with one from their enemies, such person is gone astray, lost, and kaafir……” Source: Bihar 25/362.