Men are not to mingle revealed truth with their own imaginations and traditions. In speculating on the unseen, the risk of error is indefinitely great, and that error God reproves by manifesting its falsehoods.

Add not thou unto his words - You can no more increase their value by any addition, than you can that of gold by adding any other metal to it. Take care that you do not any thing that this word forbids, nor leave undone any thing that it commands: for this is adding and diminishing in Scripture phrase.
Lest he reprove thee - Lest he try thy word by fire, as his has been tried; and it appear that, far from abiding the test, the fire shows thine to be reprobate silver; and so thou be found a falsifier of God's word, and a liar.
How amply has this been fulfilled in the case of the Romish Church! It has added all the gross stuff in the Apocrypha, besides innumerable legends and traditions, to the word of God! They have been tried by the refiner's fire. And this Church has been reproved, and found to be a liar, in attempting to filiate on the most holy God spurious writings discreditable to his nature.

Add thou not unto his words,.... To the words of God; as the Jews did, by joining their oral law, or the traditions of the elders, to the written word, and preferring them before it; and as the Papists, by making their unwritten traditions, and the sense and determinations of their church, equal to the Scriptures; and as all enthusiasts do, who set up their pretended dreams, visions, revelations, and prophecies, upon a foot with the word of God, or as superior to it; whereas that is, and that only, the rule and standard of faith and practice, and is a sufficient and perfect one; see Deuteronomy 4:2;
lest he reprove thee; that is, God; either by words or by blows, by threatenings and denunciations of his wrath and displeasure; or by chastisements and corrections for such daring pride, blasphemy, and wickedness; those who add to his words, he threatens to add plagues unto them, Revelation 22:18;
and thou be found a liar; a forger, speaker, and spreader of doctrinal lies, such doctrines as are contrary to the word of truth; not being built on that, but upon human inventions, and additions to it.

Add . . . words--implying that his sole reliance was on God's all-sufficient teaching.
reprove thee--or, "convict thee"--and so the falsehood will appear.