prêcher pour sa paroisse

To favour your own side while acting neutral

Said of anyone talking up their own interests while pretending to be even-handed about it.

Bien sûr qu’il trouve que son quartier est le plus agréable de la ville, il prêche un peu pour sa paroisse.

Of course he thinks his neighbourhood is the nicest in town, he would say that.

Origin uncertain. Presumably it started life as a plain dig at parish priests who'd naturally talk up their own congregation over the one down the road, though nobody's pinned down when it turned into a general figure of speech.