Vivekachudamani 25

Sastrasya guruvakyasya satyabuddhya vadharanam | 
sa sraddha kathita sadbhiryaya vastu palabhyate || 
(Sankaracarya's Vivekachudamani 25) 

That by which one understands the exact import of the scriptures, as well as the pregnant words of advice of the preceptor, is called 'shraddha' by the wise; by this alone does Reality become manifestly clear. 

The prescribed Vedic activities executed without inherent shraddha or resolute faith. Whatever is offered regardless of how perfectly, whatever is performed regardless of how austere, whatever is donated regardless how opulent; if devoid of inherent, resolute faith it has no value whatsoever in this world or the next, in the present life and in future lives and is known as a-sat the antithesis to SAT perpetual goodness. Without faith, the supporting cause is absent and does not create an effect in time affecting future reincarnations. 

"By rendering transcendental loving service to Supreme Lord, one automatically performs all subsidiary activities. This confident, firm faith, favorable to the discharge of devotional service, is called "śraddhā." Thus a devotee should be confident that by carrying out the injunctions of bhāgavata-śāstra, or Vedic literature that directly, rather than indirectly, describes devotional service to the Supreme Personality of Godhead, one will easily achieve all knowledge and the perfection of life. 

"Śraddhā indicates a faithful mentality, which can be analyzed in two sections. The first type of faith is a firm conviction that all the statements of the multifarious Vedic literature are true. In other words, the understanding that Vedic knowledge, in general, is infallible is called śraddhā, or faith. The second type of faith is the belief that one must personally carry out a particular injunction of Vedic literature in order to achieve his goal in life. A devotee of the Supreme Lord should thus apply the first type of faith to the various kalā-vidyās, or Vedic material arts and sciences, but he should not accept such scriptures as pointing out his personal goal in life. Nor should he carry out any Vedic injunction that is contradictory to the injunctions of Vaiṣṇava scriptures such as the Pañcarātra.