Here's what I might would write back but maybe not:
Dear Mother,
I am writing to you in hopes that you can possibly be a happy person now before it is too late. The good life is now and I am so happy not to live in fear of God, Satan, and demons anymore.
It breaks my heart how you have turned your back on your children, including your grandchildren because of your conditional love (?). With everything you know about true motherhood, how could you just flush my youth down the toilet? I have turned toward unconditional love now, and am thankful for the daily joy I find now and am thankful for better "inner peace and security" now instead of always waiting for such good things, like the apple always dangling before the horse. It's too bad you can't enjoy your children and grandchildren now fully because of your negative, critical, conditional love.
You are the only mother I know who would turn your back on your children unless they believe just like you. It truly makes me sick to my heart that you will only grow old and die unfulfilled. I think of my children and assure them always of my unconditional love and to enjoy being young and having plenty of fun and be educated in principles of goodness and reality.
You deserve to be happy, Mother. Why remain bitter and hateful? I, too, am saddened when I tell my friends how my mother is so judgmental and critical of the majority of mankind, including her own flesh and blood. How is it possible that I could have been born from someone like this?
I am returning your negative and critical book as I desire to be a much better, real person.
Love,