Note the two important qualifications: It is their daily work i.e. what they earn their living with; and it concerns leaders of men in matters of faith.

About the first point it may be remarked that Gibbon - "The Decline and Fall of the Roman Empire", that contains much information about religions and priests, among other things - tells us that the pagan religions in ancient Rome and Greece did not have priests in this sense, and that what priests do in the Christian and Mohammedan religions was done by ordinary citizens in the religion of the ancients. He also tells us, in one of his many beautiful notes - note 57 to Chapter XXXVIII -

'I have somewhere heard or read the frank confession of a Benedictine abbot: "My vow of poverty has given me a hundred thousands crowns a year; my vow of obedience has raised me to the rank of a sovereign prince." I forget the consequence of his vow of chastity.'

About the second point it may be remarked that religion is a matter of faith, not knowledge, and that in all probability - for any priest not of one's own faith, if one has a religious faith - a priest is deluded, a deceiver, a fraud, a hypocrite or a fool. This does not exclude that such a person does good and maybe mostly sincere, but if so then presumably because he is benevolent and is deceived by his own wishful thinking.