True prayer is when one endeavours not so much to ask for ones own wishes to be fulfilled, but rather to adjust those wishes to the Divine Will. Prayer is not really a group activity. Prayers made up on the spot by preachers mostly just represent the ego of the prayer.
Prayer is between the the individual and the Creative force, the essence of which should be to accept the will of God.
It is impossible to test the prayer hypothesis because outcomes would rarely aline with what the prayer thinks they need and life is made up of both good and bad fortune and I've never met anybody with enough insight to determine the outcome of prayer.
I have read of a fellow who has won the lottery I think it was 8 times and given every cent away to the needy. This fellow prayed fervently for this to happen.
