Prabhupāda: Firm conviction in the Supreme Personality of Godhead as he instructs: sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja (BG 18.66).
So after taking sannyāsa there is no other business than to take shelter of Kṛṣṇa's lotus feet. Go on. ziddha-svarūpānanda adhyāsitāṁ–means worship by pūrva tamair–means by previous ācāryas.
Prabhupāda: So it is not a new thing most of the ācāryas they accepted. Just like in the beginning all the four ācāryas namely Rāmānujācārya, Madhvācārya, Nimbārka, Viṣṇu Svāmī, they accepted sannyāsa and in that line Caitanya Mahāprabhu He accepted sannyāsa. Then further down to my Guru Mahārāja he accepted sannyāsa, I have accepted sannyāsa, so pūrvatamair mahadbhiḥ (SB 11.23.57).
(I shall cross over the insurmountable ocean of nescience by being firmly fixed in the service of the lotus feet of Kṛṣṇa. This was approved by the previous ācāryas, who were fixed in firm devotion to the Lord, Paramātmā, the Supreme Personality of Godhead.)
As previous ācāryas accepted, so I am accepting also. Mahat eh?, parātma-niṣṭhām. Full faith for developing full faith in the Supreme Personality of Godhead.