My church is having a 25th anniversary party. Can we serve alcohol, either BYOB or donated. No one will be charged for their drinks.

What country are you in ? And if the US, what is the state liquor laws ?

In Missouri and Georgia, Churches serve beer at most weddings and other events, even as fund raisers. The people having the event pay the church for the cost.

Your church should know the rules

Does your church have any written policy (constitution, bylaws, etc.) about serving hard liquor/beer/wine on the premises, even at a private church function? Will there be limits on the amount consumed? monitoring and designated drivers will be needed?

Churches serve alcohol (usually wine more than liquor) at private events all the time. But churches are no more exempt from liability if something happens either on the premises or between there and peoples' homes than any other organization or business. So it helps to send out a memo about designated drivers, types of alcohol allowed, when drinking will end, and general concern regarding responsible drinking. And be sure the food is still plentiful past the time the drinks end so that no one is drinking on an empty stomach.

For a specific answer to your question, your general location (country, state, city, etc.) is needed. Laws and regulations can vary greatly between locations.

If you are in the US, I highly suggest contacting your state liquor licensing agency. My state (North Carolina) and a couple of others I looked up have 'one time' permits to allow religious and charity organizations to provide or allow "brown-bagging"/byob at an event.