The worth of a man does not consist in the truth he possesses, or thinks he possesses, but in the pains he has taken to attain that truth. For his powers are extended not through possession but through the search for truth. In this alone his ever-growing perfection consists.

[Nicht die Wahrheit, in deren Besitz irgend ein Mensch ist, oder zu sein vermeint, sondern die aufrichtige Mühe, die er angewandt hat, hinter die Wahrheit zu kommen, macht den Wert des Menschen. Denn nicht den Besitz, sondern durch die Nachforschung der Wahrheit erweitern sich seine Kräfte, worin allein seine immer wachsende Vollkommenheit bestehet.]

Gotthold Lessing (1729-1781) German playwright, philosopher, dramaturg, writer

Eine Duplik, Part 1 (1778) [tr. Chadwick (1957)]
    (Source)

This passage (in the Scott Horton translation below) is given as the epigraph to chapter 19 of Christopher Hitchens, God Is Not Great (2007); this prominence gave it a fair amount of fame. It is identified in Hitchens as being from Lessing's Anti-Goeze tracts (1778), though strictly speaking the passage is actually from Eine Duplik (1778), a different writing by Lessing over the same Fragment Dispute of 1777-1778.

(Source (German)). Alternate translations:

It is not the truth that a man possesses, or believes he possesses, but the honest pains he has taken to get at truth, which makes a man's worth; for it is not by the possession of truth, but by the march after it, that his powers are extended, in which alone his perfection consists.
[Source (1884)]

The true value of a man is not determined by his possession, supposed or real, of Truth, but rather by his sincere exertion to get to the Truth. It is not possession of the Truth, but rather the pursuit of Truth by which he extends his powers and in which his ever-growing perfectibility is to be found.
[tr. Horton (2007)]