"Niścayāt means to have full faith in the words of Kṛṣṇa. Kṛṣṇa says, kaunteya pratijānīhi na me bhaktaḥ praṇaśyati (BG 9.31). This is called full faith, that 'Kṛṣṇa has said that if I am in devotional service, I will never be lost. Kṛṣṇa will give me protection.' Kṛṣṇa says, ahaṁ tvāṁ sarva-pāpebhyo mokṣayiṣyāmi (BG 18.66). To have faith, this is called surrender. Not that, 'Oh, Kṛṣṇa says, but may not be able to protect me.' No. Kṛṣṇa is able. He is all-powerful, omnipotent. He can give you protection. When He says that ahaṁ tvāṁ sarva-pāpebhyo mokṣayiṣyāmi, that is a fact. This is called niścayāt. 'Certainly Kṛṣṇa will give me protection.' Not that 'Because I do not see Kṛṣṇa, I do not know whether Kṛṣṇa will give me protection.' No. That word is sufficient."
(Srila Prabhupada Lecture, Bombay, April 10, 1974)