Here I am trying to be snarky, and you have to go and be all serious.

You might as well keep the argument simple, and say that just as Protestantism rejects any one anointed authority to mediate between people and God (i.e. the Pope), there is no one anointed authority to rule over people (i.e. a King). Ergo, democracy.

However, democracy is not inherently Christian, given that the ancient Greeks came up with it long before Christ.