। ॥ २१६ ॥
In plain words
What makes people trust a speaker is that he says things exactly as they are — that is where śraddhā comes from. And the Veda is Īśvara's own speech; that it says things as they are is simply not in doubt.
Close to the Sanskrit
Truthfulness of speech is, for people, the cause of the generating of faith. Since the Veda is the word of Īśvara, in its being true to fact there is no doubt.