I'm playing MM1 and temple donations have no effect at all.
After reading a bit I understand that donations will make clerics in the temple buff your party.
am I missing something? it's a bug?

In MM2, you only get the blessing some of the time; maybe MM1 works the same way?

In any case, try donating repeatedly until you get the blessing. (If you use too much money without getting lucky, you might consider reloading a save to get the money back.)

You usually have to donate lots of times.

Just to add to what PetrusOctavianus said, you have to donate lots of times to same temple. Just keep selecting donate until the bonus appears.

ah, thank you, I'll do it