I don't see fading as being deceitful. If you left any other church, job, organization, etc, all you have to do is leave. Maybe you would hand in a letter of resignation at your job. You usually make the reason for leaving acceptable to those who might hire you in the future. You don't say you hate your supervisor or the company is horrible. That is private information. You might say you are leaving for health, family or personal reasons. the rest is just plain none of their business.
You also have many personal opinions about most people. Are you obliged to tell them you think they are less than intelligent, ugly, boring, fat, hypocritical and on and on? No, that is just your private thoughts or opinions and would make them feel pretty lousy. We all hold back personal thoughts and opinions daily. Is this dishonest or just respecting the dignity and feelings of others?
The real problem starts with the fact that you are dealing with a cult. If this was not so, leaving would be a non-issue. We all join and leave various groups during our lives. Because you are dealing with a cult, you have to keep more thoughts and opinions private, as they mean to do you and your family harm by breaking up your family relationships. This is how a cult operates and your family is still operating under their control. So you hold back personal thoughts and opinions to spare their feelings and to minimize damage from the cult.
So, who is really being deceitful? The ones who want to destroy your family even though they know what they are teaching is dubious, at best? Or you, the who would love to have a totally honest and open discussion, but cannot without the cult's blackmailing consequences?
I am kind of tired of people following this cultish thinking and blaming those who won't play their games. My private thoughts/beliefs are mine alone unless I choose to share them with someone else. And that is at my discretion only. I refuse to hurt my family or others because some cult thinks they own the world, make that the universe.