Couple of points.
Crimea was annexed from the Turks in 18th century, and since that time was a part of Russia, not a part of Ukraine. However, in 1954 Khruschev gave Crimea to Ukraine Soviet Socialist Republic as a gift commemorating the union of the two peoples.
Russian Federation is a legal successor to the USSR and so has to honor the international treaties of that time. So legally, Crimea is a part of Ukraine, but culturally it is isn’t. Crimea is populated mostly by ethnic Russians, some Crimean Tartars and some Ukrainians.
Crimea is arid and has no water, except on the coastline. All the water is pumped into Crimea from Ukraine, through the narrow isthmus. So the physical existence of Crimea as an attractive piece of real estate is predicated on Ukrainian goodwill.
Western Ukraine was annexed into the USSR by Stalin. A nationalist movement, initially allied with Nazi Germany, and after its defeat, without international support, fought the Soviet occupation as a guerrilla force. Since they were, at one point, allied with the Nazis, the anti-Ukrainian propaganda calls them “Nazis” to this day. They are certainly feisty like any armed rebels would be. It is the final crushing of the UPA/Bandera movement in 1954 that occasioned the gift of Crimea.
There is another treaty that binds Russia. When the Soviet Union was breaking up, the newly independent Ukraine agreed to turn over its nuclear weapons to Russia. Ukraine also agreed that the Russian Federation Navy be permanently stationed in Sebastopol. In return, the Russian Federation guaranteed territorial integrity of Ukraine in its 1991 borders, that is, with Crimea intact in Ukraine.
So the only way to remake the map of Ukraine peacefully would be through some kind of plebiscite in Crimea and Eastern Ukraine, that would be recognized as binding by both rump Ukraine and EU. Putin is a coward, and the diplomatic battle for him is steeply uphill. I think he will rattle the saber a little bit his side of the border in hopes that the Eastern Ukrainians start a civil war. He will not however, invade across the border because the diplomatic crisis will be formidable with no upside for him.
After I went to the CIA World factbook.
World factbook information about religion in the Ukraine: https://www.cia.gov/library/publications/the-world-factbook/geos/up.html
Religions:
Ukrainian Orthodox - Kyiv Patriarchate 50.4%,
Ukrainian Orthodox - Moscow Patriarchate 26.1%,
Ukrainian Greek Catholic 8%,
Ukrainian Autocephalous Orthodox 7.2%,
Roman Catholic 2.2%,
Protestant 2.2%,
Jewish 0.6%,
other 3.2% (2006 est.)