Bhagavad Gita 9.2राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्।प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम् ॥

In this verse Lord Krishna eulogies the knowledge of Brahman very highly in order to create a great interest in the spiritual aspirants for attaining It quickly.There is neither blind faith nor faithmongering in this royal science. The truth, the sovereign,secret (the Self or the Absolute) can be directly realised by intuition or immediate perception. The science of the Absolute is the most splendid of all sciences. It is the science of sciences. Of sciences the highest, of secrets the most profoun, of purifiers the supreme is this. The knowledge of Brahman is the best purifier. It reduces the roots of all Karmas and all the Karmas themselves which have been stored up in the course of many thousands of births, into ashes in the twinkling of an eye. It destroys Avidya along with its effects. An expiatory act (Prayaschitta) cannot destroy all sins. It removes the effect of a single sin, only to some extent. Even if it is removed the effect of that sin remains in a subtle state in the mind and forces him to do sinful acts in his next birth. But the knowledge of the Self destroys quickly all the sins in their gross and subtle states that are accumulated in the course of several thousands of births along with Avidya, their cause. That is the reason why it is a supreme purifier. The causal body of the Jiva is called MulaAvidya (rootignorance)