Summary
Roman Emperor banned them in 393 AD - called them a pagan festival. Restarted in Athens in 1896.
Ancient Olympic Games started in 776 BC and were held every 4 years for over 1000 years. Until Christianity became the official religion.
Roman Emperor Theodosius I banned the Olympics in 393 AD. Reason: this festival honoring pagan god Zeus was incompatible with Christianity.
1500 years later, in 1896, Baron Pierre de Coubertin revived the modern Olympics. First modern games were held in Athens - respecting ancient tradition.
Only 14 countries participated in the first modern Olympics. Today over 200 countries compete.
Source: International Olympic Committee