Just hit your knees
The just hit your knees argument is that one may come to know god by worshipping him. That is, if a person lives by the teachings of a specific religion, then the benefits of doing so will manifest and the person may know that the religion is therefore true. The expression "just hit your knees" refers to kneeling and praying to God.
- "People find praying helps and have had answers. This week you can find out if someone does listen and care. "
In essence, this argument is an appeal to empiricism. The theist is asking the non-believer to engage in an experiment. Try X. If X results in a pleasant outcome, this means that it is a true principle to live by, and therefore this religion is a good one to follow.
A related argument is based on God answering intercessory prayers.
Fallacy of the argument
The fallacy that accompanies this argument is not in the actual theory of the argument, but in practice. When the experiment is carried out, the standard procedure for scientific experiments is not followed. In a scientific experiment, the hypothesis of the experiment must be falsifiable. That is, there must be a null hypothesis.
With this particular experiment, the test hypothesis should be "This religion is true" and the null hypothesis should be "This religion is false". When the experiment is performed, if the desired result is not obtained, a theist may often counter with a reason why the experiment went bad. For example, the experimenter did not have enough faith or did not perform the action to a sufficient standard in order for it to be valid. However, the conclusion in the case of a failed experiment should be that the null hypothesis may actually be the truth.
More generally, the fallacy being committed here is that the conclusion has been reached before the experiment has begun. In religions which promote prayer to a deity or deities, it is often said that every prayer is answered, but that sometimes the answer is "no" or "not yet", and that sometimes the answer is that there is no answer. The fallacy is that any possible outcome is interpreted as a positive result for proving the test hypothesis. There is no conceivable outcome which would imply the null hypothesis--that the religion is not true.
Scientific studies of prayer
This argument claims that one may come to know God by prayer. They separately claim that petitional prayers are answered (Matthew 7:7-8). However, analysis of many medical studies have found "no scientifically discernable effect" between prayer and health outcomes.  If prayers are not answered but believers think they are, they may be mistaken about being able to know God by prayer too.
- ↑ Try praying, advice booklet, published by There Is Hope 
- ↑ K. Masters, G. Spielmans, J. Goodson "Are there demonstrable effects of distant intercessory prayer? A meta-analytic review." Annals of Behavioral Medicine 2006 Aug;32(1):21-6. 
- Efficacy of prayer, wikipedia