Eastern Ukraine
According to a 2016 survey of religion in Ukraine held by the Razumkov Center, 73.5% of the population in Eastern Ukraine were Christians (63.2% Eastern Orthodox, 8.1% simply Christians, 1.0% Protestants, and 0.3% Latin Rite Catholics), 0.5% were Muslims, 0.3% were Jewish, and 0.3% were Hindus. Not religious and other believers not identifying with any of the listed major religious institutions constituted about 24.7% of the population. It also showed that approximately 55.6% of the population of eastern Ukraine (which in Razumkov's mapping excluded Donbass and consisted of the regions immediately to the west of it) declared to be believers, while 13.4% declared to be undecided or non-believers, and 3.5% declared to be atheist. 
A 2007 survey by the Razumkov Centre asked "Would you like to have your region separated from Ukraine and joined another state?" In eastern Ukraine, 77.9% of respondents disagreed, 10.4% agreed, and the rest were undecided. 
In a poll conducted by the Kyiv International Institute of Sociology in the first half of February 2014, 25.8% of those polled in East Ukraine believed "Ukraine and Russia must unite into a single state", nationwide this percentage was 12.5%.
History of Christianity in Ukraine
In the Union of Brest of 1596 (colloquially known as "unia"), a part of the Ukrainian Church was accepted under the jurisdiction of the Roman Pope, becoming a Byzantine Rite Catholic Church, a Ukrainian Greek Catholic Church, colloquially known as the Uniate Church. While the new church gained many faithful among the Ukrainians in Galicia, the majority of Ukrainians in the rest of the lands remained within Eastern Orthodoxy with the church affairs ruled by then from Kiev under the metropolitan Peter Mogila "(Petro Mogyla)". The Orthodox Church was made illegal (its legality was partially restored in 1607), its property confiscated, and Orthodox believers faced persecution and discrimination which became an important reason for large numbers of Ukrainians to emigrate to Tsardom of Russia following the Union. The eastward spread of the Union of Brest led to violent clashes, for example, assassination of the Greek Catholic Archbishop Josaphat Kuntsevych by the Orthodox mob in Vitebsk in 1623.