Every fool stands convinced; and everyone convinced is a fool; and the faultier a man’s judgment, the firmer his conviction.

[Todo necio es persuadido, y todo persuadido necio; y quanto mas erroneo su dictamen, es mayor su tenacidad.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 183 (1647) [tr. Fischer (1937)]
    (Source)

(Source (Spanish)). Alternate translation:

All Fools are Opiniatours, and all Opiniatours are Fools. The more Erroneous their Opinions are, the more they hug them.
[Flesher ed. (1685)]

Every fool is fully convinced, and every one fully persuaded is a fool: the more erroneous his judgment the more firmly he holds it.
[tr. Jacobs (1892)]

Fools are stubborn, and the stubborn are fools, and the more erroneous their judgment is, the more they hold onto it.
[tr. Maurer (1992)]