A faithful witness will not lie
For that would be contrary to his character as faithful; and as he will not witness to a falsehood upon oath in a court of judicature, so neither will he tell a lie in common conversation. This may be applied to Gospel ministers, who are witnesses of Christ; the Gospel they preach is a testimony concerning him, and they bear a faithful witness to the truth; nor will they, knowingly and willingly, deliver out a falsehood, or a doctrinal lie, since "no lie is of the truth", ( 1 John 2:21 ) ; the character of a faithful witness is given to Christ, ( Revelation 1:5 ) ( 3:14 ) ; who is a "witness" of his father's love and grace, of his mind and will, and of the doctrines of the Gospel relating to himself, and the method of salvation by him; and he is "faithful" to him that appointed him; nor can he nor will he lie, for he is "truth" itself; but a false witness will utter lies;
or "blow" F5 them out, and spread them abroad in great plenty; he will not stick to tell them, and, having no conscience, will utter them as fast as he can, with all boldness and confidence; for one that fears not to bear testimony to a falsehood upon oath, will not scruple to lie in common talk. Or the words, "nay" be rendered, "he that uttereth lies [will be] a false witness"; he that accustoms himself to lying, in his conversation with men in private company, will become a false witness upon occasion in a public court of judicature: such an one is not to be depended on; lesser sins lead to greater, lying to perjury. So false teachers, and the followers of the man of sin, speak lies in hypocrisy, doctrinal ones, which they are given up to believe; and such as do so are false witnesses, deceivers, and antichrist.
F5 (xypy) "efflabit", Montanus; "efflat", Junius & Tremellius, Piscator.