In terms of definition, I think there is a very extensive overlap. Most church music is sacred music, and most sacred music is church music. If you’re looking for church music that is not classified as sacred music, possibly a hymn that is set to the tune of Hey Jude, for example, or another Beatles song might be considered an instance of church music that is not properly described as sacred music, because it wasn’t specifically composed for Church use. At the other extreme, you could say that Beethoven’s Missa Solemnis is certainly sacred music, simply because it is a musical setting of the Mass, but doesn’t qualify as church music because it is appreciated, by and large, primarily as a work in the choral music genre, along with Fidelio and the Ninth Symphony, and only secondarily as a form of Christian worship.