An authority is someone who is called upon to give his expert testimony about some matter under his competence.  The credibility of an expert, of course, is directly proportional to his expertise in that area.  An athelete pushing a particular soft drink is in no better a position to offer an opinion on the product than anyone else.  On the other hand, all experts hardly agree with one another on any number of topics, religion included.

The Ad Hominem or "Against the Man" fallacy, as it is sometimes known, seeks to dismiss someone's argument or credibility on the basis of a personal attack against him instead of addressing the actual argument advanced.  The propagator of this fallacy is really seeking to exempt himself from the burden of responding to the points raised against his position by heaping contempt or ridicule on his opponent.  A variant form of this fallacy is sometimes known as tu quoque or "you're another.  The perpetrator of this kind of ad hominem attack will appeal to his opponent's guilt in order to deflect the fact that he himself is guilty of a transgression.  Another kind of ad hominem is known as "poisoning the well", where the speaker will attempt to cast doubt on the viability of an argument by directing the audience to the motives of his opponent.