Many people forget that the envelope is part of their church fund raising letter, too, and they neglect to put something enticing on the envelope that will get people to open it. You don't have to write anything complicated, just one phrase or sentence that will encourage people to open the envelope and read what's inside.
Your church fund raising letter should be as short as you can make it without sounding choppy. Get to the point as soon as you can, don't waffle on trying to avoid saying what it is you want. On the other hand, don't mention that you're asking for money in the first sentence of your church fund raising letter. Use some common sense; thank the parishioner for their time, attention, and most importantly their past support. Then talk about the reason for your church fund raising letter - because the church needs help for a specific project that the parishioners can support with a donation.
Including a postage-paid return envelope with your church fund raising letter may sound like you're giving even more money away, but it's the quickest way to get people to send donations. The less effort they have to put forth, the more likely they are to help you in response to your church fund raising letter, so show some respect for that. Make sure the only thing they have to do is put their check in the included envelope and mail it back to you.