Catholicism

Last week on visitation I witnessed to several Catholics. At the end of our 2 hour visitation I ended up debating a Catholic guy for just over an hour, thus making visitation 3 hours that day (oh, no!). The conversation began with me asking if the guy (who's a youth director at a Catholic church) if he knew for sure if he was going to heaven. He said no and that no one could know. I asked him if I could show him a verse and he said sure. I took him to 1st John 5:13 and emphasized the "know" (just like how you know your name) and "have" (present tense). The guy seemed kind of awestruck for a second. Then, as most debates.. er... uh... conversations do, he changed the subject a few times, but we always made it back to salvation and the surity of it. I asked him how long forever, eternal, and everlasting were and he agreed they were forever, eternal and everlasting. Then I asked him, if 1st John 5:13 says saved people HAVE eternal life, how can you stop something that's eternal? Anyway.... the teen that was with me said that the conversation sounded like a Kent Hovind debate (www.drdino.com). I stumped the guy numerous times by continually going to the Bible (which, he said was God's inspired Word, but doesn't give us everything we need for living), but I couldn't convince him (not that you can when first talking to someone).
My question is this: What are the best verses or best way to convince a Catholic that their belief in salvation is wrong? Do you break their trust in the church first? Do you just give them continual verses on the subjects discussed like I did (which he admitted later caused him to doubt some)? What are your experiences?