The following is based on a conversation that I had with Srila Gurudeva on the aspect of Nishtha in Bhakti. Srila Gurudeva expressed Nishtha as the summum bonum as all practices that are followed in the context of Bhakti especially under the auspices of a God-realized Spiritual master.
The material world operates on the principle of verification first and then accepting the same as a result of verification of a concept or principle. For example the process of smoking “may” result in cancer, is accepted as a universal truth based on experiments conducted with rats exposed to tobacco and then verifying the same through mathematical principles of probability and so on. However in the spiritual domain it is a reverse process. Especially in Bhakti, faith comes first and the truth is established later in the heart of a particular devotee in proportion to his faith. Hence the results obtained in Bhakti are different for different people. There is always a difference in the in-built faith system from individual to individual. The Shastras simply provide a guideline to the Ultimate through examples and evidences from the path but may not be singularly capable of complete transformation of the heart of the Bhakti who simply associates with the Shastra on his own. The Bhakta may not have the capability in the first place to connect with the Shastra in its entirety and is prone to speculate on the words mentioned in the Shastra owing to his own limited intelligence and his inability to fathom the depth of the words expressed in Shastra.
Hence it becomes utterly important to accept a Spiritual Master who is perfect. The Paramatma in the heart of a devotee will guide the devotee based on the desire of the devotee to such a master irrespective of the incapability of the devotee to identify such a master. However after having accepted such a spiritual master, it is binding on the disciple to have “Nishtha” or determined faith on the words of the spiritual master who is the seer of the Absolute. Nishtha is not “Sraddha” which is feeble and wavering and it is also not “Vishwaas” or firm belief, confidence or ordinary faith. Nishtha means undeviating and absolute faith. A person knows in his heart, guided by Paramatma that one’s spiritual master is perfect, yet may harbour doubts because of Anarthas, previous karmas and so on. In such a case it becomes binding on the devotee to cry to the Lord to free him from his illusory conception of lack of determined faith. This will only happen if the devotee repeatedly cries in his heart to Guru and Bhagawan with single-pointed sincerity.
Nishtha represents the highest ideal of a Sadhana Siddha or a person who has perfected himself from his inabilities through sustained practice of spiritual principles. A person who does not have such Nishtha should first develop Nishtha on Bhajana and Sadhana and once the Sadhana Bhajana of the devotee becomes free of such anarthas then the devotee will be automatically get guided by Paramatma within to scale the highest limits of Bhakti which is beyond the domain of all Shastras. Unless one has firm faith on Sadhguru, the Sadhana process and the commitment to attain perfection in all respects Bhakti will only remain in shadow form and the devotee will be far away from tasting the nectar of Rasa or the mood of the Spiritual world where the divine couple enjoy clandestine pastimes. The mood of Conjugal love is a mellow that has to germinate in the heart. This only germinates from the womb of Guru Nishtha and Bhajana Nishtha. There is simply no other process.