One thing you need to remember when you are sitting down to write your church fundraising letter is that no one likes to be hassled about money, so don't just start off sounding like you're begging. Talk about the activities and improvements the church is planning, and invite them to be a part of this by supporting the effort financially. You do want to get to the point fairly quickly, though, because no one wants to read for pages and pages when you can say what you want to say with your church fundraiser letter in a few short paragraphs. In your church fundraiser letter, you'll want to strike a balance between conciseness, and thorough explanation.
Begin the church fundraiser letter by greeting the person and reminding them about the church, and be sure to thank them for any past support they have given. Most people like to know that anything they have done for the church in the past has not gone unappreciated, so be sure to include this in your church fundraiser letter. As you explain what improvements the church needs to make, be as polite as you can, but also make your point clear. You need your supporters' help, and your church fundraiser letter is the one chance you get to impress this upon them.