There is a particular kind of quiet that settles over a family when a child who was raised in the faith decides they no longer hold it. It is not the same as a child who never believed. You are not introducing them to something unfamiliar. You are living with the fact that they know all of it — the songs, the verses, the shape of a Sunday — and have set it down on purpose.
That changes what a letter can usefully say. Most advice about sharing your faith assumes an audience who has not heard. Your child has heard. They can probably quote you. Whatever you write, they will read it against every conversation the two of you have already had, and they will recognise the moves.
So the letter has to do something other than make the case again.
Start by admitting what they already know
The temptation is to open gently and then work round to the argument. Your child will see that coming from the first line, and the rest of the letter will be read with their guard up.
It is better to say plainly that you are not writing to persuade them. Then keep that promise. A letter that says "I'm not trying to change your mind" and spends four paragraphs trying to change their mind costs you more than saying nothing — it tells them you will still use warmth as a way in.
If your faith matters to you, say so once, in your own voice, as a fact about you rather than a claim about them. Once is enough. They already know; repetition reads as pressure, and pressure is the thing that made the distance in the first place.
Separate the relationship from the disagreement
A lot of these letters fail because they treat the child's belief as the main subject and the relationship as a means to it. Your son or daughter can tell the difference between being loved and being handled.
Write about them. Not their position — them. What you noticed about them when they were small and still notice now. What you have watched them do well. Where you have been proud and not said so at the time. What you got wrong as a parent, if there is something, said without the qualifier that undoes it.
This is not a softening-up exercise before the real content. It is the real content. If the letter never mentioned belief at all, it would still be worth them having.
What to leave out
The argument. The verses, the apologetics, the case you have made and remade in your head. If they have left, they have already declined it at least once. Making it again in writing gives them something to refute rather than something to keep.
Anything with a verdict in it. "I hope you find your way back." "I know one day you'll understand." "I'm praying you'll come to your senses." These land as you are currently wrong and I am waiting, however gently they are meant.
Guilt about what it has done to you. Your grief over this is real, and it is not their debt to pay. A letter that makes your sadness their responsibility gives them a reason not to reply.
Anything about other people's opinions. What their grandmother would have said, what the church thinks, how it looks. None of it is between the two of you, and all of it reads as recruitment.
What it can sound like
A few lines to show the shape rather than to be copied:
You have not been a disappointment to me. I want that written down somewhere you can find it, because I am not sure I have ever said it in a way you believed.
I know what you think I want from you. I am not writing to ask for it.
Whatever you believe or don't believe, you have been one of the great goods of my life. That was true before any of this and it is true now.
I would rather have you honest with me than agreeing with me. I think I have not always made that easy.
Notice what none of them do: none of them ask for anything.
Say the practical things too
Alongside all this, say the ordinary things people leave until it is too late. Where the will is. What you want done. Which of your possessions you hoped they would take, and why that particular one. Who else should be told.
Practical detail is not a lowering of the tone. It is a kindness. It gives a grieving person something to do, and it says, without saying it, I was thinking of you specifically.
If you want a fuller guide to the shape of this particular letter, we keep one at letters to your unsaved children. And if what you are carrying is mostly the ache of it rather than the words, praying for a child who is not saved sits closer to that. For the wider case of a relative who has simply never believed, what to say to a family member who is not saved covers the different ground.
Write it before you are sure what to say
Most of these letters never get written because the parent is waiting to feel ready, or waiting for a thaw that would make the letter unnecessary. Neither tends to arrive on schedule.
You do not have to resolve anything to write it. You only have to say the true things while you can still say them — and let your child decide, in their own time, what to do with having been loved that plainly.
Write your first letter free — it takes one sitting, and it is one of the few things in this that is entirely in your hands.