Kosov was born into the Ruthenian nobility of the Grand Duchy of Lithuania in the town of Zhyrovitsy in what is today western Belarus. He became Mohyla's student and colleague, and was a co-founder of the Kyiv-Mohyla Collegium, of which he was prefect from 1631.
Kosov's tenure as Metropolitan of Kyiv coincided almost exactly with Khmel'nyts'kyi Rebellion, which began in 1648 as a Cossack uprising within the Polish-Lithuanian Commonwealth led by Hetman Bohdan Khmel'nyts'kyi allied with the Crimean Tatar Khanate. It led to the creation of an independent Cossack Hetmanate in Ukraine. It also resulted in the expulsion or killing of almost all Polish nobles, officials, and Roman Catholic clergy in the lands of present-day Ukraine, as well as widespread massacres of Ukraine's Jewish population.
But perhaps the most pivotal event of this period for Ukraine's modern political history as well as the history of the Kyiv Metropopolia was the Pereiaslav Council of 1654. Since it was in the geopolitical interest of the Crimean Tatars to maintain a state of conflict with the Polish-Lithuanian Commonwealth, they withdrew their support at several key moments, preventing Khmel'nyts'kyi from achieving a decisive victory. Looking for alternative military support, he made diplomatic overtures to both the Ottoman Empire as well as to Moscovy. The latter agreed to provide assistance in exchange for the Cossack's allegiance to tsar Alexei I.
In the negotiations that led to the "Pereiaslav Agreement" (or the "Pereiaslav disagreement", as Serhii Plokhy has aptly referred to it), neither party fully comprehended what the other thought that it was agreeing to. Rather than being the "reunification of Ukraine with Russia" portrayed in standard Russian and Soviet history writing, the two sides had very different ideas of the relationship between the two polities: Khmel'nyts'kyi thought he was agreeing to a protectorate, while the tsar assumed he was simply gaining new subjects. The cultural and linguistic gulf between the two sides was so deep that they needed interpreters even for basic communication.
The end product, however, seemed to grant many of the Cossak's demands, including the preservation of the Kyiv Metropolia within the jurisdiction of the Ecumenical Patriarchate. However, realpolitik soon took over and set in motion a major reconfiguration of power in Central and Eastern Europe. The resulting Russo-Polish War (1654–1667) led to the Truce of Andrusovo, which formally placed the territory east of the Dnipro River under the control of the tsar. Eventually, this would lead to increasing restrictions on the Hetmanate and the incorporation of Ukraine into the Russian Empire under de facto colonial status, and would even contribute to the demise of the Polish-Lithuanian Commonwealth itself.
Both Kosov and Balaban strongly resisted attempts to subordinate their Metropolia to Moscow and continued to show loyalty to king John Casimir, as can be seen on their antimensia (above).
Balaban was a strong supporter of the 1658 Treaty of Hadiach, a last-ditch effort to maintain the Hetmanate within the Polish-Lithuanian Commonwealth. Proposed by Hetman Ivan Vyhovs'kyi and written by Yurii Nemyrych (a Western-educated Nontrinitarian Protestant who converted to Orthodoxy) with the assistance of Balaban, the treaty would have, among other things, created a co-equal Grand Duchy of Ruthenia, given the Orthodox Metropolitan of Kyiv and other Orthodox bishops seats in the Commonwealth Senate, and granted the Kyiv-Mohyla Academy the status of a university alongside the Jagiellonian University of Cracow. The treaty was approved in an extremely watered-down form, and it only succeeded in inflaming rank-and-file Cossacks against the nobility and made them even more agreeable to closer ties with Russia.