Torah jurisprudence is very careful about authenticating evidence and determining the facts. Hard evidence is needed to convict a person, and therefore one witness is not sufficient in court. Perhaps he is not telling the truth; and even if he is not lying intentionally, perhaps he is imprecise or is just imagining things. Therefore a situation can be authenticated only by the testimony of two witnesses. This is not the case in gentile law where, for example, they believe a “State Witness” who is not only one witness testifying alone, but is also biased because he stands to benefit from incriminating the accused.
Our sages teach that the Jewish people are judged in heaven according to the laws of the Torah. Therefore, when the Accusing Angel, the Satan, comes forward to declare their sins, he is told: “You’re just one witness so we can’t accept your words.” Immediately his claims are put on hold and G-d’s quality of “restraining His anger” is activated to delay punishment and give the sinner a chance to repent. But there are cases in which the Accuser’s allegations are accepted: If a person also comes and slanders the defendant and relates his negative deeds, then the heavenly Accuser is no longer a sole witness to the evil deeds, but there are now two witnesses — the Accuser and the slanderer. Heaven will now deliberate on the accused’s sins and will accept the slanderer’s accusation to convict the man.
We can now explain the statement of our sages: “The sin of slander is considered equal to the three capital offenses in the Torah — idolatry, adultery and murder.” Although slander is very serious in itself, it has another drawback that does not exist in other sins: it causes that when a person's sin is spoken about, he is now in the sights of the heavenly court. Until the slander was spoken, the sin had left no effect, but when the slanderer joined the Accuser on the witness stand, an incriminating and decisive testimony was now presented to convict the accused.
Things work the same on the positive side as they do on the negative side. The Sages say: “One who answers Amen is greater than one who recites the blessing.” (Brachot 53b) Why should this be so? The one who recited the blessing initiated it, said it himself, and had in mind the blessing’s meaning, whereas the one who answered Amen just stated one word implying consent!
We can understand this according to the principle we mentioned above: one who recites a blessing is just one witness, whereas if there is another witness with him, his blessing is not just a supposition but a verified statement. When his friend says Amen, he is essentially serving as a second witness, which verifies the blessing. Therefore, the one saying Amen gets an even greater reward than the one who recited the blessing.
We see, then, that not only are two witnesses required in our legal system here on earth, but, for better or for worse, in the laws that determine spiritual matters. This is an example of how heaven’s reckoning is far beyond our simple understanding. May G-d help us merit to achieve our spiritual perfection, amid true humility before His greatness and the complexity of how He conducts His world.