The most perfidious way of damaging a cause is deliberately to defend it with faulty arguments.

[Die perfideste Art, einer Sache zu schaden ist, sie absichtlich mit fehlerhaften Gründen vertheidigen.]

Friedrich Nietzsche (1844-1900) German philosopher and poet

The Gay Science [Die fröhliche Wissenschaft], Book 3, § 191 (1882) [tr. Nauckhoff (2001)]
    (Source)

Also known as La Gaya Scienza, The Joyful Wisdom, or The Joyous Science.

(Source (German)). Alternate translations:

The most perfidious manner of injuring a cause is to vindicate it intentionally with fallacious arguments.
[tr. Common (1911)]

The most perfidious way of harming a cause consists of defending it deliberately with faulty arguments.
[tr. Kaufmann (1974)]

One injures a cause in the most perfidious manner by deliberately defending it with erroneous reasons.
[tr. Hill (2018)]