Parāçara said:—O twice-born! Thus meditating upon Vishnu as identical with himself he attained to that unification, which is desired by all and regarded him as the undecaying divinity. He forgot his individuality and was not conscious of anything. And he thought that he himself was the endless, undecaying supreme soul. And on account of this efficient notion of identity, the undecaying Vishnu, whose essence is wisdom, appeared in his mind which was wholly purified from sin.

In plain words: Parashara says that as Prahlada meditated on Vishnu as one with himself, he lost all sense of being a separate person. He felt he was the endless, eternal Supreme Soul. Because his mind was now pure, Vishnu appeared within it.