After being out for 40+ years allow me to assist in how to leave.
1. Make sure that you really don't believe in it any more. That is the key.
2. Accept the fact that no matter what you say or write to friends or elders, the impression that you will make on still-ins is absolutely zero. So don't bother to explain, they'll make even more mileage out of that than if you just walk.You say it best when you say nothing at all.
3. Accept that you will lose friends and possibly loved ones too. You will, without doubt. Accept it, or don't do it.
4. Fill your spare time with good stuff. Most of your non-working hours will have been spent on witnessy things. You'll have a big hole to fill.
5. Recognise the fact that you, as a human, are a social creature and that you need people. So get a social scene organised anyway that you can. College, sports, hobbies, whatever.
6. Don't look for an alternative religion. It'll just be another belief system to replace the last one. Forget it. If there is a God and he wants you to do something, he's pretty sure to tell you.
7. Don't become a victim. Victims can quickly become a pain in the arse and the sympathy you receive initially will dry up very quickly.
8. Learn how to have fun!