Father Zakaria's arsenal is his knowledge in not only Christian Scripture, but also the works of Islam. By knowing this, he is able to point out the inconsistencies in the religion and question many of their commonly held beliefs.
Father Zakaria's story can be found in his biography Defying Death: Zakaria Botross: Apostle to Islam. Although I am yet to read this book (I just found out about it before writing this), I am sure it will be a great read and his story an example to all of us. Let us not worry whether we will be asked if we know the Scriptures, but rather be like Father Zakaria and know not only our own Scripture but the opposing viewpoint.