Zohar, Mishpatim, pp.98b-99a
Before undertaking an in-depth inquiry into the nature of the souls of converts, there are some rules in Jewish Law that one must know. Every Jew has an inheritor who is a relative, however distant, with the right to inherit his property, and when he dies his property is never "up for grabs" since there is always someone who has the legal right to inherit it; in contrast, a convert does not have a Jewish family tree that goes back for generations, and he might die leaving no inheritors. In such a case the rule is that his property is up for grabs. Whoever grabs the property of a convert who has died and left no inheritors merits the property. The Zohar, in a famous section in which an old man, a saba, reveals secrets to Rebbe Shimon' circle, explains the esoteric meaning of this rule.
"All these holy and high Neshamot that are designated by the Blessed Holy One to go down below, as we explained, go out at specific times to play in Paradise. There they meet the souls of converts. One that takes hold of one of these souls, and unites with it, merits it, envelops itself within it, and goes out…"
The Sabba began to cry as he had done before, and he said to himself, "Old Man, Old Man, you certainly have reason to cry and pour out tears over every word that you are revealing…
"All these holy souls come down into this world to inspire human beings and to take their places as is fitting for each one of them. When they come down each one is clothed in those souls that we discussed [the souls of converts], and that is how they enter into the holy seed. It is within these garments that they are able to exist and to be effected by the things of this world. When these envelopes draw desired things from this world, then the holy Neshamot are nourished from the odors that they smell in these garments. The Nefesh of the convert is a garment that envelops the higher souls…
The Nefesh of the convert is a garment that envelops the higher souls from the levels of Neshama and Ruach, and it is through this exterior garment that envelops them that the Neshamot and Ruchot enter into the physical world. In other words, the Nefesh of the convert is the interface between the higher souls and the physical world.
The Neshama and Ruach can enter the world only when they are enveloped in a Nefesh which acts as their interface. They can only benefit from the world by the sense of smell - so to speak - the most ethereal of the senses. This they do through the interface of the Nefesh, and if they cannot use their own Nefesh, then they get to use the so-called Nefesh of the convert, which is made available to them specifically for this purpose.
It is written in the Torah (Exodus 23:9), "Do not oppress a convert, for you have known the Nefesh (soul) of a convertwhen you were strangers in the land of Egypt." The continuation of the Zohar emphasizes the need to understand this verse and the concept of Nefesh of the convert in an esoteric way, and not according to its simple meaning.
All the hidden things that the Blessed Holy One does, He has put into the holy Torah, and everything is in the Torah. The wise…are able to see a hidden thing within its garment…
He reveals a hidden thing in the Torah, but he immediately clothes it in another garment and hides it. The wise, who have many eyes, are able to see a hidden thing within its garment even though it is hidden there. They spot it when it is revealed even though it may be hidden again within its covering immediately, and they don't lose sight of it again despite its obscurity.
In several places the Blessed Holy One has already warned about converts that the people of the holy seed must be careful about them…
In other words, the people of Israel must be careful about the converts in their midst - not to oppress them or treat them harshly in any way, and to go out of their way to make converts feel at home as much as possible
Having warned about the convert in all these places, He now takes the thing out of its covering and reveals it. This is what it says, "…for you have known the Nefesh of a convert," and then it is immediately hidden with the words, "…when you were strangers in the land of Egypt."
In other words, the Sabba is insisting that we should not understand this verse according to its simple meaning: that since we were strangers in Egypt, we have the ability to empathize with the convert in our midst. The Sabba may well be doing what he is "accusing" the Torah of doing - revealing one measure and hiding two. Nevertheless, the text of the Zohar here insists that we must understand the esoteric meaning of these words, "…for you have known the Nefesh of a convert."
The appearance in this phrase of the word "known" indicates that we are talking about some sort of special "intimacy," which was hinted at by the previous description of the meeting of a higher soul with the Nefesh of a convert in Paradise. Furthermore, the Sabba reveals now that the esoteric meaning of the verse is about the relationship of Neshama (and Ruach) to the Nefesh of the convert.
It is through this Nefesh of the convert that the Neshama knows the things of this world and is able to enjoy them. The Nefesh of the convert is the interface…
That is the main point. The Nefesh of the convert is the interface. The Zohar now clarifies the point by comparing it to the cloud that is mentioned in the verse, "And Moses went into the midst of the cloud, and he went up to the mountain…" (Exodus 24:18).
What is this cloud?
This is what is written, "I have put My rainbow in the cloud…." This rainbow sent forth its garment, and it was given to Moses. And with this garment, he [Moses] was able to go up to the mountain, and through it he was able to see what he saw and benefit from what was there.
In other words, the cloud was the interface between Moses and what was being revealed on Mount Sinai.
At this point the members of the fellowship [that were present, R'Yossi and R'Chiya] prostrated themselves before the Sabba. They cried and they said, "If we had come into the world only to hear these words from your mouth, then it would have been enough."
Nevertheless, the following question remains to be asked: Why is this special interface that is created specifically to allow blemished Ruchot and Neshamot to reincarnate into the world - why is it called the Nefesh of a convert?
First, this special interface Nefesh is comparable to a convert because it also has no history or genealogy. It was created for this special purpose of serving as an interface. It does not have a previous "owner" to which it will return.
Second, this special interface Nefesh is comparable to the property of the convert who has died without any inheritors. Whoever takes hold of it, unites with it and uses it, will possess it forever, in addition to his own Nefesh. The Nefesh of the convert is like a new creation…
The Arizal has explained in Gates of Reincarnation (Chapter 7) that the Nefesh of a convert is a fourth type of soul in addition to the three main types (i.e. Nefesh, Ruach, and Neshama). The Nefesh of the convert is like a new creation, and therefore it can serve as the vehicle for a blemished Ruach to come into the world - on the condition that the Nefesh of that Ruach has already been completed.
Nevertheless, the Arizal teaches that the gentile background of a convert may continue to exert influence over a period of one or two generations. In such a case, even second or third generation Jews may be born with the Nefesh of a convert. The example of this is none other than Rebbe Akiva whose father was a convert; this explains why he did not study Torah and was an enemy of those that did for the first forty years of his life.