"There are some cases in which a person overcome with pain, his mind exhausted, grieves, mourns, laments, beats his breast, & becomes bewildered. Or one overcome with pain, his mind exhausted, comes to search outside, 'Who knows a way or two to stop this pain?' I tell you, monks, that stress results either in bewilderment or in search." (AN VI.63)
This is the classic cause for spiritual search. Many people go through their lives not caring about spiritual pursuits ... until something very painful happens to them. As the Buddha noted here, one of two things tends to happen at this point: either the person begins searching spiritually, or he becomes confused and has trouble handling the situation, often resorting to repression and other such coping mechanisms. But what can come of the spiritual search? One can find solace looking outside oneself, as in devoting oneself to a god, or one can see that one need only take refuge in oneself. In the Dhammapada (160), the Buddha said:
Your own self is
your own mainstay,
for who else could your mainstay be?
With you yourself well-trained
you obtain the mainstay
hard to obtain.
But how can we be sure of this? When we observe closely our body, our feelings, our perceptions, our mental formations, and our consciousness, we see that (Dhp 165):
Evil is done by oneself,
by oneself is one defiled.
Evil is left undone by oneself,
by oneself is one cleansed.
Purity & impurity are one's own doing.
No one purifies another.
No other purifies one.
Initially, we may need to have faith that the practice will reap such rewards. However, faith is only the catalyst. Soon, with true effort, one will experience for oneself the self-evidence of these two verses by the Buddha. One will know for oneself that they are true, thus faith is dropped in favor of knowing. This knowing is the basis of the path of Buddhism.