A Christian crucifix can be an idol. If you bow down to it thinking that the thing has power to save you it has become an idol for you. It is interesting that the very bronze snake which typified Christ on the cross (see John 3:14-15) set up by Moses became an idol at one stage and had to be destroyed. I have not got the reference handy in my head but it is in the Old Testament somewhere where this event happened - in the book of Kings no doubt.