Many apologetics have sprung up to explain away the passages in the Bible that declare that those who pray will get what they ask for - in light of the fact that this seems to not be demonstrably true. In a previous post I give the passages in question and I took on the apologetic excuse that god only answers prayers that are concurrent with his will. There are other excuses though, and one commenter continually decided to try and harp on one (even though I explicitly stated that the OP was about a different argument). I've also previously argued against other apologetics, like the argument that prayers are answered, but on god's timeline and that Jesus was only empowering the specific people he spoke to (although the latter one does not deal with this verse specifically, it's good enough for the purpose of dispelling any notions that the issue has been dealt with).
Never-the-less, I decided that perhaps I should deal with the others. So, this post will focus on the apologetic tactic of trying to interpret the phrase, "If ye have faith." This argument from the apologist focuses on the conditional phrase that is uttered in Matthew 21:21
Matthew 21:21 Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done. 22:22 And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.
So, the obvious tactic here is to claim that Jesus was not incorrect, and that those prayers that are not answered are coming from people who don't have faith - which is generally interpreted to be not having enough faith or not having the right kind of faith. How convenient - and how very post hoc. This allows the Xian to claim after the fact that an "answered" prayer fit the description and a "non-answered" prayer did not. It's the same as painting a bulls-eye around an arrow that has already been shot.
Yet, it still falls apart under investigation, as no Xians claim to have 100% success rate for prayer (that I know of at least). If any do claim this, then we only need to have them pray for something immediate (like the regrowth of an amputated arm for instance) to test and see if this person can validate their success rate claim. Invariably it will fail.
Also, this is nothing more than the no true Scotsman fallacy, writ large. I can imagine someone saying, "True Xians have their prayers answered, so if a prayer is not answered, then the Xian in question is not a true Xian." It's fallacious though, no matter how you slice it. Once again, we see apologetics that fail to answer the objections brought forth.