According to the Buddha's teaching, doubt "vicikicchā" is one of the five Hindrances "nīvarana" to the clear understanding of Truth and to spiritual progress. Doubt, however, is not a 'sin', because there are no articles of faith in Buddhism. In fact there is no 'sin' in Buddhism, as sin is understood in some religions. The root of all evil is ignorance 'avijjā' and false views 'micchā-diṭṭhi'. It is an undeniable fact that as long as there is doubt, perplexity, wavering, no progress is possible. It is also equally undeniable that there must be doubt as long as one does not understand or see clearly. But in order to progress further it is absolutely necessary to get rid of doubt. To get rid of doubt one has to see clearly.
There is no point in saying that one should not doubt or should believe. Just to say I believe' does not mean that you understand and see. When a student works on a mathematical problem, he comes to a stage beyond which he does not know how to proceed, and where he is in doubt and perplexity. As long as he has this doubt, he cannot proceed. If he wants to proceed, he must resolve this doubt. And there are ways of resolving that doubt. Just to say ' I believe', or ' I do not doubt' will certainly not solve the problem. To force oneself to believe and to accept a thing without understanding is political, and not spiritual, and not spiritual or intellectual.
The Buddha was always eager to dispel doubt. Even just a few minutes before his death, he requested his disciples several times to ask him if they had any doubts about his teaching, and not to feel sorry later that they could not clear those doubts. But the disciples were silent. What he said then was touching: 'If it is through respect for the Teacher that you do not ask anything, let even one of you inform his friend' 'i. e.,let one tell his friend so that the latter may ask the question on the others' behalf'.
not only the freedom of thought, but also the tolerance allowed by the Buddha is astonishing to the student of the history of religions. Once in Nālandā a prominent and wealthy householder named Upāli, a well-known lay disciple of Nigantha Nātaputta 'Jaina Mahāvīra', was expressly sent by Mahavīra himself to meet the Buddha and defeat him in argument on certain points in the theory of Karma, because the Buddha's views on the subject were different from those of Mahāvīra. Quite contrary to expectations, Upāli, at the end of the discussion, was convinced that the views of the Buddha were right and those of his master were wrong. So he begged the Buddha to accept him to reconsider it, and not to be in a hurry, for 'considering carefully is good for well-known men like you'. When Upāli expressed his desire again, the Buddha requested him to continue to respect and support his old religious teachers as he used to.
To continue
from the book of ' What the Buddha taught' written by WALPOLA RAHULA