"A German is someone who cannot tell a lie without believing it himself"

The text refers to the stereotype that Germans are perceived as a sincere and truthful people who have problem informing falsehoods. It suggests that the German culture puts a high worth on sincerity and speaking the fact, to the point that even if a lie is told, the teller might think it to be real. Adorno might also be suggesting that the Germans have a tendency to be self-righteous and think in their own ethical supremacy.