You can somewhat but you weren't if you feel that Christians should "own" the rainbow symbol and it's meaning by right of divine provenance (which is fine by me that you feel that way). I simply disagree that you can own a symbol like a rainbow, there's no enforcement mechanism on who owns what symbol other than how people think and interpret symbols which is why the Nazi's could successfully repurpose the symbol of life into a representation of their viscous ideology.
Symbols mean what people think they mean. You can't "own" their meaning and thus you can't steal it, you can merely convince people in one way or another to change their mind about what they think symbols mean.
But overall, my point to your point is that the stolen idea always requires an attached idea of ownership.