This is the 9621st most frequent Russian word.
христа
"Христа" is the genitive or accusative case of "Христос," meaning "Christ."
Here, "Христа" refers to the name Christ in the genitive case, indicating possessive relation to "life."
В детстве я часто слышал рассказы о жизни Христа.
In childhood, I often heard stories about the life of Christ.
In "Рождество Христа," "Христа" is again in the genitive case, showing the association of Christmas with Christ.
Каждый год мы празднуем Рождество Христа.
Every year we celebrate Christ's Christmas.
Here, "Христа" is used in the genitive case to indicate the purpose or reason tied to Christ.