I have no recollection of the Catholic in-laws ever referencing the ‘key of David’...
First line, third verse—sung every Advent since the 9th century at the latest, predating English by a good bit,
Veni, veni Emmanuel;
Captivum solve Israel,
Qui gemit in exilio,
Privatus Dei Filio.
Refrain
Gaude! Gaude! Emmanuel,
Nascetur pro te, Israel!
Veni, veni, O Oriens;
Solare nos adveniens,
Noctis depelle nebulas,
Dirasque noctis tenebras.
Refrain
Veni, Clavis Davidica!
Regna reclude caelica;
Fac iter tutum superum,
Et claude vias inferum.
Refrain