"The Christian religion, [Hume] says, cannot at this day be believed by any reasonable person without a miracle. ''Mere reason is insufficient to convince us of its veracity: And whoever is moved by Faith to assent to it, is conscious of a continued miracle in his own person, which subverts* all the principles of his understanding . . .' But of course this is only a joke. What the believer is conscious of in his own person, though it may be a mode of thinking that goes against 'custom and experience', and so is contrary to the ordinary rational principles of the understanding, is not, as an occurrence, a violation of natural law. Rather it is all too easy to explain immediately by the automatic communication of beliefs between persons and the familiar psychological processes of wish fulfilment, and ultimately by what Hume himself was later to call 'the natural history of religion'." (J. L. Mackie, The Miracle of Theism: Arguments for and against the Existence of God, 28-29)
* "Subverts" is a bit inaccurate in my view. This suggests that the understanding is being corrupted in some way, while it is completely possible to possess an uncorrupted understanding and recognize the rationality of contrary arguments and yet possess faith in Christ. Perhaps Hume used "subvert" to alert his readers to his joke, but it seems to me a more accurate word might be, "supersedes."
Faith, according to Mackie = a subversion of the understanding arising from the psychological process of wish fulfillment.
Faith, according to the Bible = the assurance of things hoped for, the conviction of things not seen. By faith we understand that the universe was created by the word of God, so that what is seen was not made out of things that are visible (Heb 11:1,3).
It's surprising how similar the definitions are. The author of Hebrews, though, goes a step further to suggest faith is the gateway to another sphere of understanding altogether, "by faith we understand..."
After reading Mackie I've gained a clearer understanding of why it's impossible to convince anyone of Christ's authenticity. Pure rationality cannot overcome the hurdles which it presupposes and therefore cannot allow faith to be an option. However, as a Christian, having been privy to the knowledge and experience arrived at through faith (but still capable of recognizing the enforced limits of pure rationality), am myself startled by the strength of my faith. Mackie suggests that he can explain what the believer is conscious of, but really he can do so only speculatively, not being in possession of faith himself. The genuine experience of faith enjoyed by most believers is never a subversion of the understanding, but rather a supersession of the understanding. In faith there is a certain assurance which a person of faith immediately recognizes as superior in authority to reason. Scripture would suggest that it is not wish fulfillment which drives faith but rather an invisible source perceived subjectively which is immediately recognizable as having greater authority concerning reality than that which can be arrived at through observation and rationality. Mackie, within the propositional universe of reason, may be able to satisfy himself with apparently obvious explanations for the phenomena of faith, but such speculations inherently deny the possibility of what is invisible and cannot of themselves arrive at the knowledge which faith itself imparts to the believer. Hume's joke may ring true within the realm of pure rationality, but scripture attests that faith itself is indeed a gift; a different and separate faculty altogether for arriving at knowledge of divine "invisible" things. True faith, i.e., faith which supersedes rather than subverts the principles of understanding, according to the Bible, is a miracle.