What kind of rhetoric device is this?

Metaphor

The question for me is which part are you asking about? The treble “white/bass black/God’s keyboard” bit is, as @janbb notes, a use of metaphor. The contrast between “treble white” and “bass black” is a use of antithesis (sometimes called “floating opposites”). And framing the whole metaphor in terms of every man being made in the image of God is a use of ethos. So the mode of persuasion in use is ethos, and it is achieved through a metaphor that operates on antithesis.

But if this is a homework thing, then “metaphor” is probably the answer your teacher is looking for.

Oh my god! Thank you both! That’s perfect! This is going to help!

(You mean “rhetorical device,” to be accurate. “Rhetoric” is a noun.)

You could also simply say “What kind of rhetoric is this?”