The following sample letter can be used as a staring point when writing "The Letter". Obviously, you will want to tailor it to your own specific situation. A great deal of detail as to why you don't think the church is "true" is probably not a good idea in the initial step. If asked then you may want to go into particulars, but if you present those kinds of items in the ice breaking letter things may turn into a fight (by using an argumentative approach) rather than a request for understanding.
Dear Mom and Dad (or friends or family),
I (or we if leaving as a couple or family) have been doing some major thinking and studying over the past few years regarding our religion. My intention in writing this is only to be honest with you, and I hope you will be open minded as you read on. Please mull it over for several days before discussing it with anyone as it may be a bit shocking at first. It will be better if some initial time is used to sort out your feelings.
I feel that growing up in the church has offered many positive benefits and opportunities. In particular, we are grateful for the emphasis on family values, good morals, and honesty. However, we have some concerns about Mormonism and raising our kids in the church. I donít want to go into details because I know of your strong belief and love for the gospel, and I respect your choice of beliefs. I want you to know that my emerging beliefs are not in vast conflict, as far as morals go, with those of the church. We have no desire to change the good standards of living that we have been taught, and plan to nourish those in the kids as well.
Basically, Iíd like to continue to grow in an environment which allows a less restrictive view of life, and I'd like our kids to have those same opportunities. I want to raise them in such a manner that they will achieve their fullest potential without having to go through some of the guilt and pressures that may be unnecessarily felt by LDS youth. I donít think that raising them in a church which claims to be "the one true church on earth", and personally not believing this, would do them any good. I want them to have the freedom to live out their own thought-out convictions. I also want to be totally honest with each other and the kids. Pretending to believe is no longer an honest option.
My personal reasons for doing this are based on a great deal of research (several years worth), and this turn of events has in no way been a hasty decision. The initial conclusions that I was drawing were very painful to me at first, but in the long run it has been a positive experience. My personal integrity has been preserved in the process.
It is not my intention to hurt you by making this choice. You are my parents, and I love and respect you both. You have done nothing to cause this change of events in our lives. We personally must follow our own paths. I hope that you can respect that. We are hoping that our relationship with the family and our friends will continue to be as good as it has been in the past. Our love for you and the family hasnít changed, and we hope you will remember this first and foremost.