Pradyumna: "By the grace of Lord Śrī Kṛṣṇa, we had the chance of being born in a Vaiṣṇava family, and in our childhood we imitated the worship of Lord Kṛṣṇa by imitating our father, and our father encouraged us in all respects to observe all functions such as Ratha-yātrā and Dola-yātrā ceremonies, and he used to spend money liberally for distributing prasāda to us children and our friends. Our spiritual master, who also took his birth in a Vaiṣṇava family, got all inspirations from his great Vaiṣṇava father Ṭhākura Bhaktivinoda. That is the way of all lucky Vaiṣṇava families. The celebrated Mira Bhai was a staunch devotee of Lord Kṛṣṇa as the great lifter of Govardhana Hill. The life history of many such devotees is almost the same because there is always symmetry between the early lives of all great devotees of the Lord. According to Jīva Gosvāmī, Mahārāja Parīkṣit must have heard about the childhood pastimes of Lord Kṛṣṇa in Vṛndāvana, for he used to imitate the pastimes with his young playmates.
According to Śrīdhara Svāmī, Mahārāja Parīkṣit used to imitate the worship of the family Deity by elderly members. Śrīla Viśvanātha Cakravartī also confirms the viewpoint of Jīva Gosvāmī. So accepting either of them, Mahārāja Parīkṣit was naturally inclined to Lord Kṛṣṇa from his very childhood, and he might have imitated either of the above-mentioned procedures, and all of them established his great devotion from his very childhood, a symptom of a mahā-bhāgavata. Such mahā-bhāgavatas are called nitya-siddhas, or souls liberated from birth. But there are also others who may not be liberated from birth but who develop a tendency for devotional service by association, and they are called sādhana-siddhas. There is no difference between the two in the ultimate issue, and so the conclusion is that everyone can become a sādhana-siddha, a devotee of the Lord, simply by association with the pure devotees. The concrete example is our great spiritual master Śrī Nārada Muni. In his previous life he was simply a boy of a maidservant, but through association with great devotees he became a devotee of the Lord in his own standard, unique in the history of devotional service."
Prabhupāda: So this is the difference between sādhana-siddha and nitya-siddha. Gaurāṅgera saṅgi-gaṇe, nitya-siddha kari' māne, se yāya vrajendra-suta-pāśa. Gauḍa-maṇḍala-bhūmi, yebā jāne cintāmaṇi, tāra haya vrajabhūme vāsa. So nitya-siddha means he has no chance to forget Kṛṣṇa. That is called nitya-siddha. And sādhana-siddha means by following the regulative principles, the rules and regulations, one revives his Kṛṣṇa consciousness. Both things are siddha because you cannot revive Kṛṣṇa consciousness without being Kṛṣṇa conscious. It may be covered. So nitya-siddha means he does not get covered by the influence of material nature about his natural devotional tendency to serve Kṛṣṇa. He never becomes covered. This is difference. Kṛṣṇa gives him chance to get birth in such a family. Just like Parīkṣit Mahārāja, that he never gets the chance of forgetting Kṛṣṇa. So one who does not get the chance of forgetting Kṛṣṇa is called nitya-siddha. This is the difference.