Among many apologists, I often hear a distinction offered between "knowing" and "showing" that the Christian faith is true. On such a view, it is supposed that one can have a warranted knowledge in the truth of Christianity wholly apart from public evidence. Such an approach has long troubled me for a number of reasons, the foremost being that I have no internal witness to the truth of Christianity -- at least not of the kind that, in my judgment, would warrant belief. It would thus be rather dishonest for me to assert such an internal witness when I have no such subjective experience to speak of.
Now, I think in principle this could be a valid approach. For instance, suppose that whenever someone became a believer, there was a voice from heaven, only heard by the new believer, that said "Welcome to the family." That, I think, would be rational warrant for affirming Christianity to be true -- even though you would need to appeal to public evidence in order to demonstrate to others that Christianity is true.
I am not prepared to lie about my own experience when talking to non-believers, or for that matter other believers. I have not had some sort of tangible subjective experience that I would consider to impart to me rational warrant for my beliefs. Yes, I believe myself to have a relationship with God (which is absolutely necessary for any believer to have) -- but when I pray, I have confidence that God hears and answers my prayer because of the public evidence I have studied. By that, I do not mean to rule out the possibility that other believers have had such an experience that would rationally warrant belief.
Answered prayer is another piece of subjective evidence often asserted to warrant belief, and again, I think this could be a valid approach in principle. However, to use it as belief-warranting evidence, one would have to demonstrate a statistical significance to answered prayer, in order to distinguish it from mere coincidence. All Christians who pray can speak of times where they have requested something in prayer where they have not received what they asked for. There are a number of explanations given for this in Scripture. For example, prayer can be hindered by sin (Proverbs 28:9, 1 Peter 3:7) or by selfish-intent (James 4:3), and sometimes God knows that what we ask for is not good for us, and often his will and purpose is different from ours. All these potential variables make it difficult to use answered or unanswered prayer as evidence for or against the Christian faith. If fulfilled prayer is to be used as evidence for the truth of Christianity, one must be able to specify a hypothetical outcome which in principle could be dis-confirmatory evidence. This makes arguing from fulfilled prayer complicated.
If one were to ask me why I myself am a follower of Jesus, I would have to say "My faith rests entirely on the public evidence." The cumulative force of the evidence for Christianity gives me a robust basis for believing Christianity to be true -- and the extent of the evidence in which my faith is grounded means that my faith is not immediately perturbed by encountering fresh counter-evidence or arguments that I have not previously been exposed to (in much the same way that a well supported scientific theory is seldom overturned by a single anomalous observation) -- or, indeed, if some of the evidence on which my faith rests turns out, in the course of time, to be less strong than I presently believe (in much the same way that the discovery that some of the evidence for the earth's vast age was weaker than I previously thought would not seriously cause me to doubt the conclusion, which would still be supported by significant other public evidence).
For these reasons, even though I ground my personal faith in the public evidence (and not in subjective internal experience) my confidence in my faith is not tossed to and fro by the shifting sands of evidence.
An objection that I frequently encounter in this regard is that basing one's Christian convictions on the public evidence diminishes the role of the Holy Spirit in conversion. But this isn't so. Rather, it is my position that the Holy Spirit uses arguments and evidence to draw men into the kingdom of God.
If the appeal to subjective personal experience does not warrant the Mormons in their belief, why should it warrant the Christian? The way I know Christianity to be true is exactly the same as how I show Christianity to be true -- by means of appeal to the public evidence.