Many owe their greatness to their enemies. Flattery is fiercer than hatred, for hatred corrects the faults flattery had disguised. The prudent man makes a mirror out of the evil eye of others; it is more truthful than that of affection, and helps him reduce his defects or emend them.

[Fabricáronles a muchos su grandeza sus malévolos. Más fiera es la lisonja que el odio, pues remedia este eficazmente las tachas que aquella disimula. Hace el cuerdo espejo de la ojeriza, más fiel que el de la afición, y previene a la detracción los defectos, o los enmienda.]

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

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 84 (1647) [tr. Maurer (1992)]
    (Source)

(Source (Spanish)). Alternate translations:

Many owe their fortune to their enviers. Flattery is more cruel than ha∣tred, in as much as it palliates the faults, which the other makes us remedy. The wise man makes the hatred of his Enviers his looking-glass, wherein he sees himself far better than in that of kindness.
[Flesher ed. (1685)]

Many have had their greatness made for them by their enemies. Flattery is more dangerous than hatred, because it covers the stains which the other causes to be wiped out. The wise will turn ill-will into a mirror more faithful than that of kindness, and remove or improve the faults referred to.
[tr. Jacobs (1892)]

Many have been made through the greatness of their enemies. Far more to be feared is flattery, than hate, since this exposes the flaws, which flattery would conceal. The man who knows makes a mirror of spite, more faithful than the mirror of affection, and envisages his shortcomings, to correct them.
[tr. Fischer (1937)]