Conversation so far:
What is the major religion of Ukrainians?
Ukrainians are predominantly Orthodox Christians.
Do they practice other religions as well?
There are also ethnic minorities that practice other religions, i.e.
Crimean Tatars (Islam), and Jews and Karaim (Judaism).
What do Orthodox Christians believe?
CANNOTANSWER.
How are religious minorities treated?
CANNOTANSWER.
What is the history of their religion?
Ukraine was inhabited by pagan tribes until Byzantine rite Christianity was introduced by the turn of the first millennium.

Question: How was Christianity introduced?
Self-contained rewrite: How was Christianity introduced in Ukraine?