A doctrine, when coming under fire, should be defendable by argument; if it is not, it is either a properly basic belief and thus rather a given of experience than a doctrine, or probably false.
But even the best of doctrines are a tough act to follow if the whole weight of a rival worldview presses against them. They are then like soldiers fighting bravely for a good cause, but fighting a losing battle.
Other members of their own army, that is, their worldview, must come to their aid. More abstractly, the single doctrine gains its full strength if it is embedded in a worldview.
Take the Christian doctrine virgin birth. You might accept it because it is written in the New Testament, pointing to the divine inspiration of Scripture; and that is a totally legitimate and acceptable reason as far as it goes. Still, the whole thing looks oddly simplistic for a doctrine of such cosmic importance. As a first reinforcement, you might therefore adduce the principle that testimonies of eyewitnesses should be accepted as true, in the absence of counterevidence. This has made the doctrine a little more palpable and real. But is this really only a matter of weighing juridical evidence? Like in any odd lawsuit?
Take a second step. Add to your principles of divine inspiration of Scripture and the general reliability of eyewitnesses the idea that God can be expected to intervene in nature, giving the Grand Miracle the air of expectancy; add to that even a reason for God to do so (namely to revert the Fall), and you have built a frontline of impressive might, because you have added meaning to truth. And yet it is not the end of the line. You might add to all these reasons that what God is doing (directly) in the virgin birth is exactly what he’s been (indirectly) doing all the time in nature. The massive doxastic stronghold you’ve built is virtually unassailable. (Intellectually, at least – but please do mind that this is just a metaphor and should be taken with a grain of salt).
Note what has happened now. The detractor can now attack any single one of those supporting principles. Doing so with the second one will be troublesome, for that at once invalidates most of received history. The other principles are internal to the Christian worldview. Of course, the adversary can dismiss it wholesale, which relegates the investigation to the level of whole worldviews. If he seeks to refute one or more principles singly, he will have to do so on the ground of Christianity; and that is a debate every Christian should welcome, if he is interested in deepening his faith and to purge it of the dross of unwarranted assumptions.
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