It is a sin to believe evil of others, but it is seldom a mistake.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 5, § 23 (1916)
    (Source)

Variants:

EVIL. That which one believes of others. It is a sin to believe evil of others, but it is seldom a mistake.
A Book of Burlesques, "The Jazz Webster" (1924)

Evil is that which one believes of others. It is a sin to believe evil of others, but it is seldom a mistake.
Chrestomathy, ch. 30 "Sententiae" (1949)