The following extracts are from Knox’s A Vindication of the Doctrine that the Sacrifice of the Mass is Idolatry.
All worshipping, honouring, or service invented by the brain of man in the religion of God, without his own express commandment, is idolatry.
On 1 Samuel 13
Disobedience to God’s voice is not only when man does wickedly contrary to the precepts of God, but also when of good zeal, or good intent (as we commonly speak), man does anything to the honour or service of God not commanded by the express word of God, as in the matter plainly may be espied. For Saul transgressed not wickedly in murder, adultery, or like external sins, but saved one aged and impotent king (which thing who would not call a good deed of mercy?); and permitted the people, as is said, to save certain beasts to be offered unto the Lord – thinking that God should therewith stand content and appeased, because he and the people did it of good intent. But both these Samuel called idolatry: ﬁrst, because they were done without any commandment of God; and, secondly, because in doing thereof he thought himself not to have offended. And that is principal idolatry when our own inventions we defend to be righteous in the sight of God, because we think them good, laudable, and pleasant. We may not think us so free nor wise, that we may do unto God, and unto his honour, what we think expedient. No! the contrary is commanded by God, saying, “Unto my word shall ye add nothing; nothing shall ye diminish therefrom, that ye might observe the precepts of your Lord God;” which words are not to be understood of the Decalogue and moral law only, but of statutes, rites, and ceremonies; for equal obedience of all his laws requires God.
On Exodus 20: 24-26
This rule being prescribed before the establishment of the ceremonial law, which appointed altars much more costly, intimates that, after the period of that law, plainness should be accepted as the best ornament of the external services of religion, and that gospel-worship should not be performed with external pomp and gaiety. The beauty of holiness needs no paint, nor do those do any service to the spouse of Christ that dress her in the attire of a harlot, as the church of Rome does: an altar of earth does best.