Composer: J.S. Bach
'Bist du bei mir' works well as either an instrumental or vocal performance. It's very adaptable for any combination of instruments as well -- piano, organ, acoustic guitar, harp, and you can also add flute or violin for the melody line if you have such a musician playing for your wedding. I've sung it in its English translation, which is also quite lovely and very appropriate for weddings. I'll include both the German and English text below.
Bist du bei mir,
If Thou art near when life is closing
Ah, how serene,