Eden, he needs to immerse in the spiritual “river of fire,” this “immersion” is needed to remove any memories of this world, and any mindset connected to physicality, since, it is impossible for him to inherit his place in Gan Eden as long as he remembers anything that has to do with the physical world.
Similarly, when he goes “from strength to strength,” i.e., from one level to a higher level in Gan Eden, like the verse says (Yeshaya 66:23): “every month on Rosh Chodesh, and every week on Shabbos, all flesh shall come to bow before Me [Hashem],” showing that every Shabbos and Rosh Chodesh there is a spiritual ascent for all souls to come closer to Hashem, he needs another “immersion” in the “river of fire,” so that he should even forget his previous level of understanding that he attained in the Lower and Higher Gan Eden.
The purpose of forgetting his current level of understanding is so that it will not disturb him when he enters an even higher level of Gan Eden to experience an incomparably higher level of understanding of Hashem.
An impure person can only enter the Beis Hamikdash after he purifies himself by immersion in the water of a mikveh. So too, the soul cannot enter the holy levels of Gan Eden without purifying itself through immersion in the “river of fire.”
This explanation of what is this “river of fire” will be discussed further in the maamar.
