I agree that it's common for non-Christian elements to be co-opted into Christian culture, but in this case it's the other way around. Johann/Johannes/John can be very clearly traced etymologically to Latin [i]Iōhannēs[/i] to Greek [i]Ἰωάννης[/i] to Hebrew [i]יוֹחָנָן[/i], roughly meaning 'God is gracious.' The name's of Danish origin, of course, but it comes into Danish because of Christian influences and distinctly arises [i]from[/i] Christian mythos.