I think that the fundamental way in which we know Christianity is true is through the inner witness of the Holy Spirit. I do not think that arguments and evidence are necessary in order for faith to be rational, or for you to know that God exists and has revealed himself in Christ. So I would say that the fundamental way we know Christianity is true is through the witness of the Holy Spirit and reason and argument then can confirm the Spirit's witness. The person who has good apologetic arguments passes, in a sense, a double warrant for his faith. He has the warrant provided by the Holy Spirit, and then he has the double warrant provided by argument and evidence. But should he lack the argument and evidence he can still be warranted just on the basis of the Holy Spirit. That's knowing Christianity to be true.