He received his master's degree in 1989 in dogmatic theology with thesis "Treatise "De libero arbitrio et de gratia" of Ivan Paštrić (1636–1708)", and on June 23, 1997 doctoral degree with thesis "Theologian Franjo Ksaver Pejačević (1707–1781) – Features", with Ivan Golub as supervisor.

[9] After media in Croatia and Bosnia and Herzegovina, the Islamic Community of Bosnia and Herzegovina as well as some NGOs expressed their dismay of Košić's act,[10] he gave an interview for Sarajevo Catholic Weekly magazine in which he stated that he doesn't believe that Kordić was a war criminal, but was rather a victim of political games regardless of ICTY verdict and all evidences that it was based on.

[12] Bishop of Dubrovnik Mate Uzinić stated in an interview for Jutarnji list that salute Za dom spremni cannot be acceptable to Church.

[13] On 21 December 2015, Košić wrote a Facebook post referring to the possible coalition agreement between Social Democratic Party (SDP) and Bridge of Independent Lists (MOST) in which he called leader of MOST Božo Petrov "a sucker and a traitor that throws his party into the jaws of the communists", and leader of SDP and at the time incumbent Prime Minister Zoran Milanović "a thief and a fraud".

After the media reported about his post, general public expressed its dissatisfaction which resulted in Košić deleting his Facebook account.

[14][15] On 15 August 2016, on the mass celebrated for the Assumption of Mary, Košić, during his homily, urged people to vote on 2016 parliamentary election "for those who will rid us of yugo-communist heritage and carry out lustration."

He also referred to the trial to Josip Perković and Zdravko Mustač who were found guilty by German court in a first-instance verdict for murder of Croat nationalist émigré Stjepan Đureković, stating that "not only two members of secret service (UDBA) were convicted in Germany but the entire yugo-communist system that killed Croatian patriots abroad as well.

Košić wrote that Plenković brought entire country in "grave difficulties by entering a coalition with his yesterday's enemies", adding that Plenković "disregarded city of Sisak in which HDZ's fraction struggles were used by the town's new "comrade mayor" (referring to social democrat Kristina Ikić Baniček) to get elected".