In my apologetics works, I’ve found that if you can
accurately paraphrase a passage and remember at least the book and chapter, it usually does the trick. If someone challenges you on it, you’ll at least know where to look it up (and it’s always helps to highlight these verses in your Bible as well).
For example, if you are arguing for the fact that there is such a thing as mortal and venial sin, you only need to remember “1 John 5” and then paraphrase the passage to the effect that John states unmistakably there that there are some sins that are deadly and some that are not. If your interlocutor is skeptical, open up your Bible to that chapter where you have verses 16 and 17 highlighted:
Of course, it is sometimes more effective (and always more fun) to use his Bible, but I doubt he’ll have those particular verses highlighted for you.