I think the Christian delusion is harmful if for no other reason than that it weaken one's critical thinking skills. If faith is the basis for what one thinks then anything can be believed. It also adversely impacts us in polls that bolster the delusion in others, in donations to faith-based causes that are harmful, in TV, radio, and book buying habits that grant spokespersons for the delusion a bigger voice than warranted, and in voting patterns that place deluded people in power who in turn cause harm to individual people, one's particular nation, and the world at large.
I think that there is a serious problem. In Christianity there is no reason to believe everything, especially stuff that comes out of your own head. Christianity is about trusting God and building a relationship with God on God's Terms not your own.
Debunking Christianity: Quote of the Day, By Yours Truly ;-)