Takfir or takfeer (Arabic: تكفير takfīr) is a controversial concept in Islamist discourse, denoting excommunication, as one Muslim declaring another Muslim, or any individual, as a non-believer (kafir). The act which precipitates takfir is termed mukaffir. Contemporary formulation and usage of the term have their roots in the 20th-century Islamist theorist Sayyid Qutb's advocacy of takfirism (doctrine of excommunication) against the state or society deemed jahiliyah (state of ignorance and disbelief).
This problem will not be solved by debating over the minutiae of whether "calling someone a kafir makes someone a kafir, so Sunnis are a kafir." Of course, you can discuss it, and it could be good to discuss it historically and religiously.
However, it is an excuse being put forward for a viewpoint, and if this excuse is disproved, another one will be presented.
The problem here is mindset, in particular,
(a) lack of understanding of the Prophetic message and how the Prophet (S) reached out to all people rather than excluding people to form an elite group,
(b) arrogance and egotism - thinking that one is part of the small, elite, saved group while everyone else is wrong floats one's ego, and is one of the subtler traps of Shaytan,
(c) a misunderstanding of the nature of Allah. Allah, as the creator of the universe, including the human being, is not so petty to kick people out of the ummah because of a handful of historical errors.
Sometimes also (d) some individiuals are benefitting from these ideologies, such as if they get followers or fans from spreading them and appealing to people's sense of clannishness.
To be fair there may also be (e) backlash against the Sunnis who call Shiis kafir and use that as an excuse for violence.
Surely we can understand that just as it is wrong for Sunnis to declare Shiis kafir, it is wrong to declare Sunnis kafir. We should treat others as we would like to be treated.
So basically these are the things that really need to addressed. A person who has humility before Allah and is not using their religious identity to bolster their own nafs or ego will not take it upon themself to decree masses of sincere people as kaafir.
Sometimes this takes time, such as years, and it won't happen in one or two sessions.
1. Sunnis are Muslims like us and it is completely wrong to claim that they are Kafirs.
2. Sunni are following different sects e.g. Hanafi, Shafi'ee, Maliki, Hanbali, Salati, Wahabi, Sufi, Barelwi, Deobandi etc.
There are also many Sunni sub sects. All of them Muslims even those extreme groups who claim that Shia are Kafir. We believe that those who call us Kaafir are Muslims but misguided.
3. Abu Talib is a great believer in Islam and protector of the Prophet (SAWA) and this fact is been admitted by mass majority of Sunni scholars. Small minority among Sunni Muslims claim that Abu Talib was non Muslim because of the enmity against imam Ali (AS).
4. Many Sunni scholars have written books about Abu Talib and his great status in Islam e.g. Ahmad Zaini Dahlan the Imam of the Haram of Makkah.
Addendum: It may depend on whether one is using the term for matters of religious law (such as marriage) or in a theological sense. It also depends on how one is using the word "agnostic".
In a theological sense, the Qur'anic term "kafir" is used for those who deny the existence of God or the blessings of God, or God's message, not for those who are genuinely uncertain.
Insofar as an agnostic person neither affirms nor denies these things, and really doesn't know, they are somewhere in the middle (neither mu'min nor kafir).
Some people call themselves "agnostic" because they dislike organised religion (for instance, they had a bad impression of it growing up) but they don't reject the idea of the divine. So this is not theologically the same thing as a "kafir". What it means is that they do not identify as part of a specific religious group, and also are not sure what they believe personally. Oftentimes, this type of agnostic is a genuine seeker of the truth and is in flux since they haven't found answers to their questions yet.
However some people use "agnostic" as a loose synonym for atheism or just not caring enough to think about spiritual matters, and so this type of person could be called a kafir, meaning that they are fully engrossed in the material world and heedless of anything else.
Today there is a tendency to use "kafir" casually as a synonym for "non-Muslim". However, because we live in a sensitive era, it is good to be sensitive and thoughtful about how we use language (not the least because some people are killed on grounds of takfir). This is why when working with Qur'anic texts, I personally try to avoid translating "kafir" as "disbeliever" (or, even worse, "infidel") and instead go for a more literal translation along the lines of "deniers".
Also, it is good to remember that the Prophet and Imams were open to having dialogue with people who were not sure about their beliefs. If they were just called "kafirs" and rejected, Islam wouldn't have spread.
An agnostic who would be someone that says they cannot confirm God's existence, nor can they reject God's existence would technically not be considered a "believer". One who doubts God existing or not cannot be classified as a believer of a religion, and hence they would be considered a disbeliever.
May the Almighty guide us all to the right path.
Thank you for your question. Persecution has been an unfortunate aspect of being a Shia throughout most of the Muslim world for a long time. Indeed the Ma'sumin (as) themselves faced these issues and so the best way to deal with such things is to take their (as) example. Remain balanced and faithful to the religion of Islam and empathise with your fellow Shia in times of difficulty. Pray for the Shia and for the return of Imam al-Mahdi (af) and turn to Allah for the solution. Where you can help alleviate the difficulties, which is sometimes even with a kind word, then be that source of help and encouragement. And know that God will never forsake those who are faithful and will recompense all that have suffered on the path of truth.
May you always be successful
By testifying that there is no God but Allah and the Muhammad (saw) is the messenger of Allah a person enters the fold of Islam and cannot be classified as a Kafir, even if they do not agree with the issue of Imamah despite clear proofs and evidence.
May you always be successful
We should not cut ties with any relative even if they are bad to us and call us Kafir.
Our behavior should prove to them and other relatives our real identity and whether we are Kafirs or not.
Ahlul Bayt (AS) taught us to mix and treat our relatives and tribes with best manners. People wll then know the real teachings of Ahlul Bayt by seeing the good practice of the Shia.