"Whatever you ask in prayer, believe that you have received it, and it will be yours" (Mark 11:24) is reckless and false, preying on both religious believers' idealistic hopes as well as the psychological impulse to rationalize away religious failures.
Ever heard the phrase "it's easier to ask for forgiveness than beg for permission"?
Obviously this is talking about just doing things you want, without having to explain yourself or go through the (sometimes difficult or uncomfortable) formalities and courtesies that might otherwise be required beforehand.
I've always thought there's a kind of tangential connection to this sort of reasoning whenever I've thought about Biblical verses like Mark 11:23-24, and what I believe is the underlying logic and appeal of this: "Truly, I say to you, whoever says to this mountain, ‘Be taken up and thrown into the sea,’ and does not doubt in his heart, but believes that what he says will come to pass, it will be done for him. Therefore I tell you, whatever you ask in prayer, believe that you have received it, and it will be yours."
Whether this was actually said by the historical Jesus, as the gospel implies, or if it was just a secondary teaching of the Christian community that was only later ascribed to Jesus, I think the crux is the same: it's easier to lure people in with the promise of fantastic abilities and miracles, but then to let their self-doubt and tendency to rationalize things away take care of things when it doesn't come true, than it is to admit that faith isn't actually as fantastic and powerful as it's made out to be, or that God isn't really looking out for our interests and health and well-being like we might otherwise want to believe.
I think we can also make a connection here with other things throughout the history of religion, too, like eschatological predictions.
For example, it's easier to just go ahead and say "the end is near" — or, in more traditional Christian garb, "the kingdom of God is near" — and then when this fails to take place in the way that it was supposed to, to just let people rationalize it away: for example, that it was just a "spiritual" end or spiritual kingdom, or that it meant "near" relative to God's perception of time and not ours, or that God changed his mind or decided to give people more time to repent, or whatever the explanation might be.
Again, similarly, when it comes to prayers being answered or Christians' supernatural abilities, this is also rationalized away in various ways: the idea that there were some unstated conditions/limitations for this, or that this may have been the case in the past but no longer applies today, or — as something that may be cunningly built into the original prediction itself — that it's simply a matter of "not having enough faith." However, none of these three explanations likely represents the true intention of the original promise. Even elements of the third explanation in the original here probably weren't intended to truly limit fulfillment in the way some people might think.
So, in effect, whether we're talking about claims of miraculous supernatural/prayer abilities or end-times predictions, in either case it's a win-win for religious figures looking for a following: they have everything to gain (mainly the credulity of believers and their enthusiasm for the fantastic and supernatural) and little to lose (because of believers' tendency to only focus on the positive and not any failures, rationalizing the latter away).