I think that you know what you need to write. I find that most times I am overwelmned with words and I can never stop writing.
Tell them the truth. Tell them you love them and that you are not leaving them, but that you do not agree with 'their truth'. That just because you do not believe as they do that you are not leaving them. That you will always be there for them, but there is a road that YOU must follow and it does not follow theirs at this time.
That true love is long suffering and selfless and that you hope that they are able to show that love during this time toward you. That you will always show it to them despite your seeking your own truth and your own journey.
Your words will be best. Give them the truth. Ask them to respect your views as you respect theirs. Ask to be able to disagree without being disagreeable. That even though you don't see eye to eye on everything that they will be able to show you real love as described in the Bible.
Good luck!