Most of my readers probably associate “trust, but verify” with Ronald Regan. He used it in reference to negotiating with the Soviet Union during the Cold war. But, ironically, it was actually an old Russian proverb. Regan got it for one of his advisors with a strong Russian background and used it effectively.
As to its meaning, the best I have read is from a “Nancy C. Walker” in a comment in answer to this question on Quora.com: “What does the proverb ‘trust, but verify’ mean?” She wrote: “Trust but verify means that you believe the person but check the facts anyway. It’s often used when someone is saying something happened. You trust the friend to be telling the truth but you verify the facts before acting on the information.”
It may come as a surprise to some that the principle of trusting but verifying is biblical. Most gospel preachers, from the earliest, have admonished their hearers to “not just take my word for it” but to verify it by searching the scriptures. This is following the example of the Bereans (cf. Acts 17:11). Four times in the King James New Testament the exact expression, “two or three witnesses” is found:
Matthew 18:16 But if he will not hear [thee, then] take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
2 Corinthians 13:1 This [is] the third [time] I am coming to you. In the mouth of two or three witnesses shall every word be established.
1 Timothy 5:19 Against an elder receive not an accusation, but before two or three witnesses.
Hebrews 10:28 He that despised Moses’ law died without mercy under two or three witnesses:
Jesus adhered to this principle: “If I bear witness of myself, my witness is not true.” (John 5:31) Jesus is not saying that he was not telling the truth about himself, but (according to the well-established principle), His testimony alone was insufficient to establish His claim to Messiahship. Anyone could make the claim, but it needed verification. He then proceeds to give his “witness list.” (1) John the Baptizer (vv. 32-35). (2) The works (miracles) the Father gave him to do (v. 36). (3) The word of the Father through Scriptures (vv. 37-39).
Jesus and his apostles each did not ask anyone to accept and pass on their subjective claim of receiving word from God, without verifying it first. They verified their claims with multiple witnesses – the many miracles which they did along with the Old Testament scriptures.
Jesus invokes the “two or three witnesses” principle in correcting the private sin of one brother against another. If the offended party is unable to correct the offender, by himself, he is told to take one or two witnesses with him in another attempt. This ultimately protects both the offended and offending parties. It protects the offending party from being railroaded before the church by a sole accuser. It protects the correcting party from coming before the church with unsubstantiated charges against a brother: “Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother. But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established. And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican. (Matthew 18:15-17)
Paul told Timothy to not receive (accept) an accusation against an elder unless there are two or three witnesses. (1 Tim. 5:19). In view of all else the Bible says about establishing things by two or three witnesses, I would think that this would apply to bringing an accusation against any brother or sister in Christ. Sometimes all it takes to smear a good name is for one person, for whatever reason, to bring an accusation against a brother or sister.
Even otherwise good people are too quick to give credence to, and repeat, damaging information without seeking verification from other witnesses to the alleged incident(s). I suspect that many, if not most, of us, including myself, at one time or the other, have accepted and passed on damaging information received from a single source. It is so easy to do. News media reporters know how easy it is to get burned by a report that they failed to check out before going to press or on the air.
Not only is the principle biblical, it is also prudent. It is always possible that a lone accuser could be mistaken about what he thinks he saw and heard, even honestly so. It could be that the initial reporter was not unbiased. It happens every day. This is why lawyers are allowed to cross examine witnesses in courts. Because humans are involved, it is always a possibility that an injustice could be done, even when two of three witnesses are involved, but it is a high probability when the testimony of a lone witness is accepted.