West Germanic is the branch of Germanic behind English, Frisian, Dutch, Low German, High German and Yiddish, as distinct from North Germanic, which produced Old Norse and the Scandinavian languages, and from the extinct East Germanic branch whose only substantially attested member is Gothic. Wayne Harbert's survey sets out the shared innovations that define the branch and the disputes about its internal divisions.
Its religious importance is that the conversion of northern Europe was carried out in these languages. Old English produced the earliest large vernacular Christian literature in Europe outside Greek and Latin. Old Saxon produced the Heliand, a gospel harmony in alliterative verse. Old High German produced glosses, the Wessobrunn Prayer, the Muspilli on the end of the world, and the Merseburg charms.
That last item is worth stating carefully. The two Merseburg charms, written down in the ninth or tenth century in a Latin manuscript, name Germanic deities and describe a healing performed by them, and they survive as an isolated fragment with nothing beside them. Almost everything else in this branch that touches the older religion was written by people describing what they were in the act of replacing.