Tell the person you will continue to study the matter further on the condition that they will study as well. Tell him the verses you will be studying and invite him to direct you to some passages to which he will refer. It may be that you do not know as much about the subject as you thought. During your study, you may find a better way to approach the discussion. If so, admit your error.
Nothing builds confidence like honesty.
The easiest way to maintain a proper attitude in conducting Bible studies is to put yourself in the other's place. Seek to understand before you seek to be understood . What may seem elementary to you may be hard for another to understand. A doctrine which seems ridiculous to you may be a vital part of your prospect's faith and value system. Imagine someone coming to your house and telling you your understanding of the Bible was wrong.