One thing you need to remember when you are composing your church fundraising letter is that no one likes to be badgered for money, for any reason, so it's not a great idea to go straight for the wallet. Start your letter out by greeting the person and reminding them about the church, thanking them for any past support. Most people who give money via church fundraiser letters are not particularly looking for recognition, but they do like to know that anything they have done for the church in the past has not gone unappreciated.
Take some time to talk about the activities and improvements the church is planning, and invite them to come and see for themselves, but don't go on for too long. A fundraising letter shouldn't bore people to death, because you want them still to be reading by the time you get around to the point of your church fundraising letter - donations. When your supporters hear about what the church is planning, they already know those things cost money, so don't be afraid to be clear about what you're asking for, in a polite and humble tone. If you include a self-addressed envelope in with your church fundraising letter, you'll get even more donations from your church fundraising letter, as it makes things very easy for the supporters.