Final answer:
Poland was the country that adopted Christianity in 966, became part of the Polish-Lithuanian Commonwealth, and was established as a republic in 1918, after regaining independence following World War I.
Step-by-step explanation:
The country that was founded and adopted Christianity in 966, became part of the Polish-Lithuanian Commonwealth, and later became a republic in 1918, is Poland. The adoption of Christianity was a significant event, aligning Poland with the cultures and religious practices of Western Europe. This move was also an important factor in the country's later participation in the creation of the Polish-Lithuanian Commonwealth. Poland then resurfaced as a sovereign state after the end of World War I, reclaiming its independence under the Treaty of Versailles.
Throughout history, promoting a state-sponsored religion defined many regions, often leading to conflicts and wars. Europe notably experienced strife due to religious differences, particularly between Catholic and Protestant states. Poland's choice of Christianity had long-standing implications for its national identity and the regional dynamics of Europe.