I guess my strongest disappointment is that my father is completely heart broken by the decision.
I realize you will be puzzled when I say the person had to leave. You find peace and comfort in your religion but for some of us it does not work that way. We feel deeply responsible, from the depth of our very being, for every word we utter. In church we are supposed to say things like “I believe in God…” But for me personally to say I believe in something means that I have personally experienced the existence of that thing or person/Person. For many, many years I said it to please the people around me, and because I wasn’t quite sure if maybe God did exist, but finally I could no longer do it. I felt like I was lying. I had no choice but to deconvert. Now that I am an atheist and have personal integrity on the intellectual level, I have the deep peace Paul and Jesus and Isaiah tell about in the Bible.
I hope your family member who has left the faith will be able to respect you for your faith. Some are unable to do so. But you can help keep the peace in the family and make this transition more harmonious if you support and love your family member unconditionally. If you find this post helpful, maybe it will also help your father. I wish only peace and harmony for you and your loved ones, whether religious or not.