We conclude Chapter 2 of Tanya.
While all souls come from the same Source, their journey through the spiritual worlds until they enter their bodies is different for every person. When bringing a soul down into this world, the parents are able to assist the soul in this world by having pure thoughts at the time of intimacy. However, though they can assist the soul through the purity they have at the time of intimacy (and their following the laws of Family Purity), their thoughts do not change the nature of the soul they bring down.
If a soul is a great soul in any case, no thoughts of any kind can affect it in the physical world to change its status. What can be changed through their thoughts however, is the garment that surrounds the soul and which allows the soul to connect with all matters in the spiritual and physical worlds while it is present in a body. It is this garment that will affect the soul throughout its life in this world. Even a great soul can be born to unworthy parents whose thoughts were not holy at the time of intimacy (and who did not even observe the laws of Family Purity as they should.)
