By Allah 'Yes'.
1.Abi Basir narrated,
Abi Ja'far[i.e., Imam Bagbir-a.s.-] say,
"Whoever thinks that they[i.e., Sunnis] are not Muslim, has lied".
(Source, Al-Kafi, V2, P25)
2.Sofiyan ibn As-Semt narrated,
'A man asked from Aba abdillah(a.s.)[i.e., Imam Sadiq] about Al-Islam[i.e., Islam] and Al-Iman[i.e., faith] ... . Then Imam(a.s.) answered," Islam is the apearance that people are upon it, [that is] testifying that there is no god but Allah; He is union; there is not any partner for Him, and [believing in] the doing of Salah, and payig of Zakah, and Hajj, and fasting of the month of Ramadhan, So, this is Islam".
(Source, Al-Kafi, V2, P24)
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Sunnis Could Go to Paradise
1.There is a Hadith from Imam Sadiq(a.s.) on Al-Kafi in which Imam(a.s.) says that those who die while do not have an Imam, and are not awar of Wilayah and leadership of 12 infallible Imams[i.e., the truth of Imamah has not been made apparent to them ], and do not hate Ahlul Bayt[i.e., Lady Fatima and 12 Imams], will go to paradise, if their good deeds are more than bad ones.(See, Al-Kafi, V3, P247).
2.Muhammad ibn Al-Fuzayl narrated from Imam Sadiq(a.s.) and he from his forefathers(a.s.), from Imam Ali(a.s.) who has said the following,
" Indeed, Paradise has 8 gates... our Shi'its and lovers will go to Paradise, through the 5 gates, and through the one of those 8 gates, other Muslims those who testify that there is no god but Allah, and do not have any hate from us, i.e. Ahlul Bayt, in thier hearts, will go to Paradise. "
(Source, Bihar Al-Anwar, V69, P159)
According to Quran, Prophet Ibrahim(a.s.) after his Prophethood and Messengership and after some trials, was chosen for Imamat by Allah. So Imamat is greater than Prophethood and Messengership.
Quran, 2:124
And when his Lord tested Abraham with certain words, and he fulfilled them. He said, 'Indeed, I make you an Imam for the people.'
But it should be said that according to our beliefs, because Prophet Muhammad(sawaws) was both an Apostle and Imam, so he(sawaws) is greater than 12 Imamas of Ahlul Bayt(a.s.) and is the best of the creatures.
Anyways, according to authentic Hadiths of our 12 Imams mentioned previously[ two first Hadiths in this post ], those who reject Imamat of Ahlul Bayt, because of either ignorance or obstinacy, are not Kafir and are Muslim, i.e. they are pure, their blood is Haram, their Ghibat(الغيبة) is Haram, their Zabihah is Halal, marriage with them is Halal, etc.
Waffaqakallah brother.
