Firm Principle

came across this as I was preparing for Sunday School this week, quoted in the monumental King and Webster Holy Scripture: The Ground and Pillar of our Faith:

Let this be a firm principle: No other word is to be held as the Word of God, and given place as such in the church, than what is contained first in the Law and the Prophets, then in the writings of the apostles; and the only authorized way of teaching in the church is by the prescription and standard of his Word.
From this also we infer that the only thing granted to the apostles was that which the prophets had had of old. They were to expound the ancient Scripture and to show that what is taught there has been fulfilled in Christ. Yet they were not to do this except from the Lord, that is, with Christ’s Spirit as precursor in a certain measure dictating the words. For by this condition Christ limited their embassy when he ordered them to go and teach not what they had thoughtlessly fabricated, but all that he had commanded them [Matthew 28:19-20]…Yet this, as I have said, is the difference between the apostles and their successors: the former were sure and genuine scribes of the Holy Spirit, and their writings are therefore to be considered oracles of God; but the sole office of others is to teach what is provided and sealed in the Holy Scriptures. We therefore teach that faithful ministers are now not permitted to coin any new doctrine, but that they are simply to cleave to that doctrine to which God has subjected all men without exception. - John Calvin (emphasis added)

How hard is that? And yet how often is it done?