In three places, the Old Testament calls for an inflexible requirement of at least two or three witnesses to convict and punish someone of a sin that qualifies as a crime according to the divine moral law, not the legal constructs of human governments. The first time at least two or three witnesses are prescribed, the numbers two and three are not mentioned, though executing someone for murder based only on one person's testimony is prohibited. Then, Deuteronomy reiterates this ethical requirement twice in a manner that clearly treats it as not applying solely to executions for the specific capital sin of murder. More than three witnesses is not problematic, of course. Anything less than a minimum of two or three is condemned if someone is to have the proper penalty for their sin imposed, but there could be more than three who come forward.
Numbers 35:30—"'"Anyone who kills a person is to be put to death as a murderer only on the testimony of witnesses. But no one is to be put to death on the testimony of only one witness."'"
Deuteronomy 17:7—"On the testimony of two or three witnesses a person is to be put to death, but no one is to be put to death on the testimony of only one witness."
Deuteronomy 19:15—"One witness is not enough to convict anyone accused of any crime or offense they may have committed. A matter must be established by the testimony of two or three witnesses."
Portions of Deuteronomy 13, 17, and 22 address in various ways how other evidences besides literal eyewitness testimony are valid, so aside from strict logical reasons why other evidences could have an equal or greater (but still fallible) epistemological weight than the hearsay of witnesses, the Bible itself does not shy away from other means of evidential support for a charge. None of the demands for multiple witnesses prescribe that people assume someone else is telling the truth if they have another alleged witness agreeing with them, but that as confrontation escalates, two or three witnesses should be involved whenever possible. Jesus, it might surprise some to find out, appeals to this concept in a context entirely divorced from legal accusation and punishment. The scenario is that of one person trying to compel an unrepentant sinner who at least calls or considers themself a follower of God and Christ to relent, with Jesus detailing progressively aggressive steps to take.
Matthew 18:15-17—"'If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over. But if they will not listen, take one or two others along, so that 'every matter may be established by the testimony of two or three witnesses.' If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.'"
Reading chronologically as the books appear in modern Bibles, this reference Jesus makes would mark the first time that the New Testament touches on having/needing two or three witnesses when dealing with interpersonal affairs where one party has either sinned or supposedly sinned. Paul adds two more such references to the New Testament total. In 2 Corinthians 13, Paul quotes Deuteronomy to a Gentile church to reinforce the severity of him needing to chastise the congregation in Corinth more than twice. If there was no transcendent moral weight to needing this many witnesses, it would be pointless for Paul to bring the idea up at all.
2 Corinthians 13:1-2—"This will be my third visit to you. 'Every matter must be established by the testimony of two or three witnesses.' I already gave you a warning when I was with you the second time. I now repeat it while absent: On my return I will not spare those who sinned earlier or any of the others,"
Next, Paul insists in 1 Timothy 5 on two or three witnesses as a requirement for seriously considering a charge against a church elder, something obviously rooted in the concept of witnesses introduced in Numbers 35 and then affirmed as applicable for even broader scenarios in Deuteronomy 17 and 19. This time, however, while he certainly invokes the principle, he does not directly use the wording of Deuteronomy other than the phrase "two or three witnesses". The connection is still plain in light of the two aforementioned books in the Torah.
1 Timothy 5:17-21—"The elders who direct the affairs of the church well are worthy of double honor, especially those whose work is preaching and teaching. For Scripture says, 'Do not muzzle an ox while it is treading out the grain,' and 'The worker deserves his wages.' Do not entertain an accusation against an elder unless it is brought by two or three witnesses. But those elders who are sinning you are to reprove before everyone, so that the others may take warning. I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favoritism."
This crucial passage [1] from Paul's epistles actually does quote Deuteronomy, but not on the matter of witnesses. Paul quotes Deuteronomy 25:4 a mere verse before he says to not act against an elder without two or three witnesses, an Old Testament prescription about the treatment of farm animals. Here, like with his quotation of the same verse in 1 Corinthians 9, he points out that if even agricultural animals should be allowed to benefit from their own labor as with an ox eating from the grain it treads, so too should humans benefit from their own labor, all the more since we are the higher animal. Then he mentions needing two or three witnesses concerning action against church elders. However, the absence of a direct quote changes nothing about the clear connection between the concept of two or three witnesses and the same requirement for witnesses in Mosaic Law. And when this threshold of accusers is reached, as would have to be the case with all matters of logic and justice, one should not spare any elder out of personal attachment or reverence, hence why Paul insists on utter impartiality.
Using the very words of Yahweh's Torah law, Jesus and Paul in fact emphasize the applicability of things like the obligation to rely on at least two or three witnesses/evidences outside of the originally specified context. In this case, the Old Testament context was explicitly about formal charges of criminal sins and the administration of the only legitimate penalties for those exact wrongs. The New Testament affirms that this principle is relevant to wider circumstances than legal proceedings. Whether at an all-encompassing level (such as dealt with in Matthew 5:17-19 and 1 Timothy 1:8-11) or that of various precise issues, and whether someone looks to pure logic or strictly logical facts about Biblical doctrines, there is no escape from the fact that the moral relativism entailed by Rabbinic Judaism and evangelical "Christians" cannot be true, with one moral standard for Gentiles and another for Jews or different standards at different points in history. At least, the could not be the case unless God was to have changed, which is rejected entirely by both the Old and New Testaments (Malachi 3:6, James 1:17).
Logically, the nature of an action would be morally mandatory or vile for all people in a given situation with respect merely to ancestry and nationality (if morality exists, that is), and neither side of the Bible ever actually says otherwise. Morality is universal according to both testaments, and the Law provides the details of morality. Neither Jesus nor Paul claims that the Law was some arbitrary contrivance of God, a pseudo-moral (morally binding, but in a relativistic way) way to culturally distinguish Israelites from Gentiles, or a temporary measure until Christ. Something like confronting or pushing back against someone on the basis of two or three witnesses would not have its moral nature changed based on who someone is, and it is not just the concern of those with societal power to punish sin.
To be universal, though, an obligation also must not be only for people devoted to God even if they represent diverse backgrounds. It is not just people committed to Yahweh of any national or racial background who should fulfill obligations like relying on two or three witnesses/evidences when proceeding with charges against a person, either when it comes to legal sentencing or organizations like the church. With certain exceptions that by their very nature cannot be for all individuals at all times (Leviticus 19:23-25, Deuteronomy 25:17-19), every human is righteous or wicked (or a mixture of both) based on their conformity to the same moral standard. If Judeo-Christianity is true, that standard was not created when God revealed laws to ancient Israel, but it was communicated to a particular group of people through the Law. Jesus and Paul alike act exactly as they should if this was the case by promoting the Law in its previously unstated but logically necessary ramifications.
[1]. After all, it addresses not only the ongoing universality of the Law contrary to moral relativism and non-theonomist "Christian" ethics, but also the inclusion of Luke's gospel among the writings of "Scripture" according to the New Testament (quoting Luke 10:7), the sharp distinction between God and Christ as their own entities, and the importance of impartiality for acting in a logically and morally consistent manner.