First, that you actually have heard what the opposite side has said or is saying.
Second, that your address their specific claims calmly, thoroughly and preferably with some sympathy for the almost universal desire not to be embarrassed. If you want to persuade, you will have to be very careful not injure the pride of the opposite party.
Third, you need some undisputable facts backed up by some easily accessed sources so that your debate partners won't have to take your word for it.
He applies these to Da Vinci Code in the rest of his post, and appropriately warns readers: "Unprepared, you can confirm the worst stereotypes about Christians as intolerant and ignorant."
While this post doesn't really give us anything new, it does remind us how to conduct ourselves. Blessings.