Why shouldn't you just ask someone if they're a Christian when witnessing?

Les explains that witnessing helps from the Navigators or Campus Crusade admonished students not to just ask someone, 'Are you a Christian?' because invariably people will say, 'Yes.' Instead, ask, 'How or why should God let you into His heaven?' Les says this is the best way to start. It puts the person on a soapbox and lets them express their own reasons. He has heard many reasons people give for why God should let them into heaven, and they invariably overlook Romans 3. This is the first step of faith on the road to salvation: recognizing that in ourselves we are lost, like sheep without a shepherd, separated because of the fall of Adam, the federal head of the whole human race. Scripture says everyone has come short of the glory of God, so we cannot enter God's heaven unless God does something on our behalf.