Christianity was a body of religions on Earth that were based on the teachings of the 1st century Human Jesus Christ. Over the centuries since the initial founding of Christianity in the first century, it had become one of the dominant religious forces on Earth.
Many Christian traditions and celebrations were observed well into the 24th century. Christmas was one of the most observed festivals aboard Federation starships including the USS Enterprise and USS Enterprise-D. (TOS episode: "Dagger of the Mind"; TNG comic: "Spirit in the Sky!")
Federation starships and facilities were also equipped with a non-denominational chapel which could be used by crew members and visitors to worship, as well as a place where marriages and funeral services could be conducted. (TOS - My Brother's Keeper novel: Constitution; TOS episode: "Balance of Terror") As of 2265, there was a non-denominational Christian chapel on Starbase 47, ministered to by Father McKee. (VAN novel: Harbinger) Miles O'Brien went to services conducted by Fitzpatrick. (DS9 novelization: Trials and Tribble-ations)