Except earlier it said to have no idols. The cross is an idol. You can appreciate a sacrifice without using the tool that caused such sacrifice as a form of worship. If you rather jumped in front of you and died to a gun shot, he sacrificed his life to save you and you would be appreciative. Would you then wear a gun necklace around your neck to show you love your dad and the sacrifice he made for you? By sanctifying his murder weapon?