An excerpt from What is the Reformed Faith: A Message from the Orthodox Presbyterian Church:
"There would be no such thing as a Reformed church today if God had not sent the great Reformation during the Protestant Reformation of the sixteenth and seventeenth centuries. But the Reformation was not only a mighty work that came from God; it was also effected through men. That it was God’s work did not make their work easy. It was the Reformers, therefore, who taught us to understand that the work of reforming the church is not finished. They said, “Ecclesia reformata semper reformanda est”: the church that is Reformed is always reforming.
"A faithful Reformed church is therefore a church that is constantly striving to think and act, to believe and live, according to the written word of God. “The purest churches under heaven are subject both to mixture and error” (WCF, XXV:5). Also, some have degenerated horribly. It follows, then, that there is but one antidote to degeneration, namely, continued diligence. It is for this reason that the vows taken by ministers in the OPC include the promise “to be zealous and faithful in maintaining the truths of the gospel, and the purity, the peace, and the unity of the church, whatever persecution or opposition may arise unto you on that account” (Form of Government, XXIII)."
From What Is the Reformed Faith: A Message from the Orthodox Presbyterian Church, Digital Edition, p. 35, Thomas E. Tyson and G. I. Williamson, (Willow Grove, PA: The Committee on Christian Education of the Orthodox Presbyterian Church, 2012).