Christian sacrament (7)
I believe the answer is:
baptism
'christian sacrament' is the definition.
(I know that baptism is a type of christian sacrament)
This is all the clue.
(Other definitions for baptism that I've seen before include "Christening ceremony" , "Initiation; christening" , "Religious ceremony" , "Christian sacrament that symbolises admission to the church" , "Christian sacrament requiring pouring of water" .)