I changed the subject in the title slightly, from Christian Faith to Christianity, since the former violates one of my thoughts: Christianity is called "a faith" for Protestant reasons, not Catholic ones. Religion is much better, much more traditional, much more Catholic. Religion is about following a rule (religio), and that is how many of the Fathers of the Church described it. Our rule is the Grace of Christ, of course, in contradistinction from the Law of Moses. Our rule is not faith, as perhaps I've said before, although it is for Protestants. The problem with equating faith and Christianity is that it overplays the former and tends towards fideism. Catholicism insists on the role of reason. You must judge for yourself which is better. BTW, the Fathers also referred to Christianity as 'the way', 'the philosophy', 'the discipline', and many other things, which are instructive for us.
I ended up in the first post on this having established that Christians are morally better than non-Christians. But this is a far cry from proving the truth of Christianity. At best we prove only that it is the best fit for man, the most natural, etc. If you take the 'most natural' and run with it, then your argument is something like,
1. There is nothing arbitrary in the world (i.e. it is given by God).
2. Man has a nature.
3. That which (best) suits his nature is a part of nature.
4. Christianity suits his nature (best).
5. Therefore, Christianity is given by God.
Even if all of this is true, it does not mean that I have sufficient grounds for it to be 'justified belief.'
Certainly the most problematic from an epistemological standpoint is (1). (I came across a wonderful quote from St. Clement of Alexandria (or perhaps Justin Martyr?) about this in my reading last night.)
The best object for apologetics is (4), and that was what I was working on in the previous post. Nevertheless, you see how much else is required for the proof.
I would asset that (2) and (3) basically sum up the whole Summa of St. Thomas.
Now, even if you grant (1), (2), and (3), you still have some work to prove (4). Think in this context about Humane Vitae, for sake of example. Pope Paul argued that contraception tends to objectify women. That is a (4)-type argument. The incredible unhealthiness of homosexuality is another example. Proponents of homosexuality, then, tend to argue that it is not something intrinsic to homosexuality that is unhealthy, but the extrinsic social reception of it. Total lies, but you see the logic. Of course, some homosexualists then turn against (2) whenever they are beaten back on (4).
It is a healthy sign when a religion attempts to promote health and happiness. But it is not easy to determine the best root to this. It used to be thought that diet cola was better for you than regular. You hear you cannot and then you can take too much Vitamin A, B, C, etc. You hear that the pill causes cancer and that masturbation is a good prostate cancer deterrent. You hear that the traditional family promotes health and well-being, and that vacations are good for you too - try doing both together! Mormons appear to be right about coffee, but wrong about red wine (at least a glass of it). Exercise is good for you - but I have a strong feeling that the lifespans of Olympic athletes is briefer than the average. Observing the Sabbath is good for psychological health, family relationships, etc., but so too is not loosing your job because you refuse to work on Sundays...
All of that pertains to love of self, you'll observe. Christianity is only about that in a secondary sense. Primarily it is about love of God and then love of neighbour. You can see how these primary commands are related to (4) in some sense. You cannot prove that God blesses you and fulfills your nature when you honour Him - even though it is true. Saying that prayer is good for one's blood pressure is not quite the same thing, although even it is a part of (4). How can Christians argue both that it is the most natural religion and yet it orients us to the world to come, and thus involves some sort of hate of the world? It can, and it does in two ways: a) a good moral life makes for happiness in this life and in the world to come, b) hate of the world means not forming unhealthy attachments. Of course, (b) must mean be interpreted in such a way as to allow for some deviations from the reductive interpretation of (4) - i.e., to include things like asceticism and martyrdom. It must, in other words, comprehend a hierarchy of values - whereby it is better to die than to deny Christ, suffer ill-health than sin.
Next we'll have to discuss the Christian way of life in its specifics.