Only one who has a complete trust in the Vedic scriptures gains the knowledge offered by them. The kind of trust is indicated by a simple story as follows.
Once in a hot and dry place, with no sign of rains for a few years and no clouds for months together, a पर्जन्य-याग, a ritual for rains, was organised. Many people had gathered in their stride, some for the sake of dharma, others due to relatives, friends, or just a change from routine, having nothing better to do. Not one of them was expecting something "unbelievable" to happen, thanks to their modern education, etc. Most of them were thinking as to there being no meteorological prediction of rain, the temperatures are to soar further. Most of them came in the lightest dress they could wear.
As the ritual progressed, it had begun to get dark due to clouds, and then it started to drizzle. By the end of the ritual, rains were coming down heavily. Although people were happy to see the rains, they wondered how to reach home with just the light clothes that they had on. Interestingly though, the priests who were officiating the homa ritual had come in prepared, with umbrellas and raincoats! They were doubtlessly sure that it will rain. That is the trust in the Vedic scriptures, shraddhA.
Similarly, only one who has श्रद्धा in the शास्त्र as प्रमाण, the right means of knowledge -- nay, the only right means of knowledge, gains knowledge, and thereby attains moksha.
ॐ तत् सत्।