It seems to me that it would be a very good test of idolatry (or lack of it) to remove the flag and see what happens. It seems to me that anyone who gets angry and insists the sanctuary must include the flag might be flirting with idolatry. Not necessarily conscious, willful idolatry, like bowing down to an idol or something, but idolatry in the sense of elevating a human symbol to absolute status alongside the symbols of the cross and the bread and wine and the Bible (as a symbol of God's Word).
Yeah... I agree with Roger, and remember not-so-fondly my own attempts at removing the American flag from our worship space. I compromised and allowed it to remain, as long as it was placed in the corner (rather than front and center) and it was shortened so the Christian flag stood above it. I would still like it removed - and have been considering doing so at least during Lent - but honestly just don't want to deal with it right now.
Anyway, I mostly just wanted a place to keep track of Roger's post.