Seerah

To prove the religious dimension, we have the hadith of Rasulullah (s):

"Whoever obeys 'Ali, obeys me, whoever obeys me, obeys Allah, whoever disobeys 'Ali, disobeys me, whoever disobeys me, disobeys Allah" [Kanz ul Ummal, hadith numbers 32973]

This hadith is absolutely explicit, obedience to Hadhrath 'Ali (as) is unconditional, it is on par with obedience to Rasulullah (s) and Allah (swt). Hence any disobedience to him, IS RELIGIOUS because it is deemed disobedience to Allah (swt).

Rasulullah (s) said:

"'Ali is with the Qur'an and the Qur'an is with 'Ali, the two shall not separate until they meet me at the Fountain of Kauthar" [Kanz ul Ummal hadith number 32912].

"'Ali is with the Truth and the Truth is with 'Ali" [Kanz ul Ummal hadith number 33018].
These two ahadith make it clear that every decision that Imam Ali (as) takes is Haqq and is supported by the Holy Qur'an and hence will ALWAYS be a religious decision. In other words if he declares war on Mu'awiya, it is the truth supported by the Qur'an, not influenced by political decision making.
If these hadith are not sufficient then we also have this clear hadith of Rasulullah (s) who said to his companions:

"Verily among you will be one who will fight for the meaning of the Qur'an in the same way that I fought for its revelation. People asked will that be Abu Bakr or Umar? Rasulullah (s) replied 'No, but he who is mending my shoes, that person was 'Ali" [Kanz ul Ummal hadith number 32967].

This hadith is absolutely explicit every Jihad of Hardhat 'Ali (as)'s is in defence of the Qur'an, to protect it from misinterpretation.

Mu'awiya's opposition to the Imam made him a baghi

Mu'awiya's opposition was clearly religious because it was a direct challenge the Head of the State. This can be proven from the hadith of Rasulullah (s):

"O Ali! Soon a rebellious group will fight against you, you will be on the truth. Whoever does not support you on that day will not be from us" [Kanz al Ummal, by Ali Muttaqi al Hind quoting Ibn Asakir, hadith number 32970]

The very fact that Mu'awiya rebelled against the Ul IL Umr Ali (as) (you deem to be the 4th Rightly Guided Khalifa) clearly proves that it therefore was a religious dimension, hence Rasulullah (s)'s referral to this group as "Baghi".

Mu'awiya's open rebellion to the Khalifa in violation to the Qur'an clearly proves this was a religious matter, and Imam Ali (as) was entitled as Ul il Umar to quash his insurgency. Imam Ali (as)'s actions were religious and in accordance with the dictates of the Holy Qur'an (Yusuf Ali's translation):

"[al-Hujurat 49:9] If two parties among the believers fall into a fight, make ye peace between them: but if one of them transgresses beyond bounds against the other, then fight ye (all) against the one that transgresses until it complies with the command of Allah; But if it complies, then make peace between them with justice, and be fair: For Allah loves those who are fair (and just)".

This verse is absolutely clear that it is a religious duty to fight baghis (rebels), of which Mu'awiya was the Head. So Imam Ali (as)'s decision to fight Mu'awiya was RELIGIOUS and was supported by the Qur'an.

Fighting Imam 'Ali (as) is on par with fighting Rasulullah (s)

The unnamed writer is absolutely right. Imam Ali (as) did not call them kaffirs, but fighting Imam Ali (as) is on par with fighting Rasulullah (s). This is proven by the testimony of Rasulullah (s):

Zaid bin Arqam narrates:

"Allah's Messenger (peace and blessings of Allah be upon him) said regarding 'Ali, Fatima, Hasan and Husain (Allah be pleased with them all): I am at peace with those with whom you make peace and I am at war with those whom you make war" (Sunan Ibn-I-Majah, English translation by Muhammad Tufail Ansari, Volume 1 page 81; Fada'il al-Sahaba, by Ahmad Ibn Hanbal, v2, p767, Tradition #1350; al-Mustadrak, by al-Hakim, v3, p149)

Perhaps we should ask the author:

1. What is your opinion about this hadith?

2. What is his position of one who is at war with the Prophet, Muslim or Kaafir?

Rasulullah (s) said:

"Ali is the door of forgiveness, whoever enters it is a momin, whoever leaves it is a kaafir" [Kanz ul Ummal, Ali Muttaqi al Hind hadith number 32910]

This being the case what opinion should I hold on one who rather than enter the Gate turns his back on it and attacks it?