| Clarke's Commentary on the Bible Two witnesses - One might be deceived, or be prejudiced or malicious; therefore God required two substantial witnesses for the support of the charge.
Gill's Exposition of the Entire BibleAt the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death,.... The idolater found guilty was to be stoned; two witnesses were sufficient to prove a fact, if three the better, but, on the testimony of one, sentence might not be pronounced. Aben Ezra observes, that some say, if two witnesses contradict two other, a third turns the scale and determines the matter; and others say, that two who are wise men will do, and three of others; and because it is said "at the mouth" of these witnesses, it is concluded, that a testimony should be verbal and not written; should not be recorded, neither in pecuniary cases nor in capital ones, but from the mouth of the witnesses, as it is said "at the mouth", &c. at their mouth, and not from their handwriting (e): but at the mouth of one witness he shall not be put to death; so careful is the Lord of the lives of men, that none should be taken away but upon full and sufficient evidence, even in cases in which his own glory and honour is so much concerned. (e) Maimon. Hilchot Eduth, c. 3. sect. 4.
Geneva Study BibleAt the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
Wesley's Notes 17:6 Witnesses - Namely, credible and competent witnesses. The Jews rejected the testimonies of children, women, servants, familiar friends or enemies, persons of dissolute lives or evil fame.
Matthew Henry's Concise Commentary17:1-7 No creature which had any blemish was to be offered in sacrifice to God. We are thus called to remember the perfect, pure, and spotless sacrifice of Christ, and reminded to serve God with the best of our abilities, time, and possession, or our pretended obedience will be hateful to him. So great a punishment as death, so remarkable a death as stoning, must be inflicted on the Jewish idolater. Let all who in our day set up idols in their hearts, remember how God punished this crime in Israel. | 
