To be one with the Supreme Lord means
Srimad-Bhagavatam
SB Canto 3
Actually, to be one with the Supreme Lord means to be one with the interest of the Lord. Becoming one with the Supreme Lord does not imply becoming as great as the Supreme Lord. It is impossible. The part is never equal to the whole. The living entity is always a minute part. Therefore his oneness with the Lord is that he is interested in the one interest of the Lord. The Lord wants every living entity to always think about Him, to be His devotee and always worship Him. This is clearly stated in Bhagavad-gītā: man-manā bhava mad-bhaktaḥ (BG 9.34). Kṛṣṇa wants everyone always to think of Him. Everyone should always offer obeisances to Kṛṣṇa. This is the will of the Supreme Lord, and devotees should try to fulfill His desire. Since the Lord is unlimited, His desire is also unlimited. There is no stoppage, and therefore the service of the devotee is also unlimited. In the transcendental world there is unlimited competition between the Lord and the servitor. The Lord wants to fulfill His desires unlimitedly, and the devotee also serves Him to fulfill His unlimited desires. There is an unlimited oneness of interest between the Lord and His devotee.