Christians in Russia are called Orthodox Christians rather than Catholics. Their faith is different from the Roman Catholics in that they do not believe religion on Earth evolves. The Roman Catholic Church has updated its doctrine to fit new scientific discoveries as well as converts in different lands. The Russian Orthodox Church still has the exact same beliefs as when it was begun. Over time, the two faiths have become widely separated in their basic beliefs.
One of the most notable differences is in the architecture of churches. Roman Catholic buildings are often in the shape of a cross inside. Many have rounded domes over the center area where the altar is located. These churches generally have bell towers. Some of these towers are square, and others are steeple shaped. These familiar features have been present in nearly all Roman Catholic houses of worship. The Russian Orthodox churches are distinct because they have onion domes.
There is no definitive answer as to why the Russians originally opted to have onion domes on their churches. Many rumors have been put forth, but no historical facts have been found. It has been noted that in 1050 the Saint Sophia Cathedral was rebuilt in Novgorod. This appears to be the first church built with an onion dome. Historical rumor has it that the style was copied from Byzantine architecture, but no historical record has been found to substantiate this claim.
While no definitive answer is available as to where the Russian Church came up with the onion dome, it has become a widespread architectural feature of Russian Orthodox churches and cathedrals. St. Basil's on Red Square in Moscow is only one famous example of this type of unique architectural feature. Trinity Cathedral in St. Petersburg and Annunciation Cathedral in Voronezh are two more famous onion domed cathedrals worthy of mention.