If one is a thoughtful and diligent student, one does not need to read with a constant mental guard up. If one has an established grid for thinking (for the Christian, this should be a biblical grid), noticing fallacious writings will be easy. One cannot learn something that is in contrast with something one already takes to be truth. A trained mind detects logical fallacies easily. An established mind is an automatic filter of truth. The Christian should be feeding one’s mental grid constantly with Scripture. Then, she or he can relax and glean from all good reading.
In fact, a disposition to be opposed to another’s writing doesn’t help learn nor detect fallacies. It only leaves thoughts looming in the mind, not dealt with. This can sometimes be more perplexing to a person than if the thought was simply given a serious consideration. When one takes the time to glean from a person with one’s guard down, that person is able to digest the information properly in order to sift through truth and and untruth. And, perhaps, if one encounters a foreign truth often enough, it will eradicate an untruth from the mind that would not have otherwise been eradicated.