The friendship of Shostakovich cast a brilliant light over my whole life and whose spiritual qualities captured my soul once and for all time.

This quote is saying that the friendship of Shostakovich had a very positive effect on the speaker’s life. The speaker is also saying that they were mesmerized by the spiritual qualities of Shostakovich, and they will remain forever captivated by them. In other words, this friendship is something that the speaker will always remember and cherish.