Why would you be concerned over anyone, including a Christian, asking you about your beliefs and giving you a pitch for their own? The only reason is if you are not completely comfortable with your own thoughts and actions and do not want to be challenged. Christians believe this is God himself working in you, encouraging you to recognize the right ways of living he has gifted you with in your heart. The mere existence of Christians is a reminder of this, which you would prefer not to have to tolerate.