Unswerving faith in the words of the saints, the spiritual master and the scriptures is called śraddhā.
This śraddhā qualifies the devotee to commence bhajana or devotional practice.
A person who doubts the words of the saints, the spiritual master or the scriptures can’t experience anything tanglible as a result from his devotional practice, therefore the saints and the scriptures have adviced us to do bhajana, consisting of hearing and chanting, while keeping unswerving faith in the scriptures and in the guru.

´Niścayātmikā buddhi’, fixed intelligence or determination, is described in the Bhagavad Gītā (2.41).

vyavasāyātmikā buddhir
ekeha kuru-nandana
bahu-śākhā hy anantāś ca
buddhayo ’vyavasāyinām

Those who are on this path are resolute in purpose, and their aim is one.
O beloved child of the Kurus, the intelligence of those who are irresolute is many-branched.

Śrīla Viśvanātha Cakravartīpāda comments as follows on this verse and thereby gives the following definition of niścayātmikā buddhi:

mama gurūpadiṣṭaṁ bhagavat kīrtana smaraṇa caraṇa paricaraṇādikam etad eva mama sādhanam etad eva mama sādhyam etad eva mama jīvātuḥ sādhana sādhya daśayos tyaktum aśakyam etad eva me kāmyam etad eva me kāryam etad anyaṁ na me kāryaṁ nāpy abhilaṣaṇīyaṁ svapne’pīty atra sukham astu duḥkhaṁ vāstu saṁsāra naśyatu vā na naśyatu tatra mama kvāpi kṣatir ity evaṁ niścayātmikā buddhiḥ—

“The devotional service my Guru has given me, consisting of hearing, chanting, remembering and serving the lotus feet of my beloved deity is my practice, my goal and my very life. Neither in the stage of sādhana, nor in the state of perfection will I be able to give up this order. This is what I desire, this is my duty and I don’t want to do anything else, not even in dreams! It may make me happy, it may make me miserable, it may liberate me from material life, it may not liberate me from material life, all this doesn’t matter to me! This attitude is called niścayātmika buddhi.” 

This is also the kind of determination of a person who is dedicated to the service of Śrī Rādhā, which is the most blissful and most delicious thing existing in the kingdom of devotion:

“The service of Śrī Rādhā is my goal, it is my practice, it is my very life, I am absolutely unable to give up the service of Śrī Rādhā. It may make me happy, it may make me unhappy, I may take birth in any kind of womb, I may go either to heaven or hell as a result of my activities – let my only duty and activity be the practice of rādhā-dāsya, consisting of hearing, chanting, remembering or serving the deity of Śrī Rādhā!”

Radhakunda Mahanta Sri Srimat Ananta das Babaji