The verb "be" is in present subjunctive mood. Basically, the sentence says that a witness to a miracle must give some testimony such that if the testimony were false (that is, it was invented and no miracle happened at all), it would be even more unbelievable than if it were true (that is, the claimed miracle actually did happen).
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contrapositionStill unresolved.... Could you give me some instances?I have never seen a miracle, nor testified to one. I don't have a single instance to give you from any personal experience.