The phrase rule of faith does not appear in the Bible. It was first used in a statement by the early church writer Tertullian in his On Prescription Against Heretics. The rule of faith is the set of standards that define a religion. Biblical Christianity holds the Bible to be its only rule of faith. The rule of faith may be different for different groups. In some cases, the standards are similar. In others, what may seem similar actually presents a vast and significant distinction.
Rules of faith in most religions rely on something other than or in addition to the Word of God, thereby denying the sufficiency of Scripture. Since the earliest days of Christianity, this heresy has survived and flourished. The rule of faith among the Gnostics of the first century was based on the Scriptures plus a mystical knowledge gained only by those who had achieved a higher plane of enlightenment. In direct contradiction to the Word of God, the Gnostics taught that salvation comes not by grace alone through faith alone (Ephesians 2:8–9) but by divine knowledge or some inner light possessed only by those of elevated spirituality.
Evangelical Protestants hold to the Bible alone as their rule of faith. This reflects their belief in the doctrine of the sufficiency of Scripture, which declares that the Bible alone is adequate to guide the Christian in all matters of faith and practice. According to 2 Timothy 3:16–17, the Scriptures are profitable to make the believer “thoroughly equipped for every good work.” If we are thoroughly equipped by the Word of God, nothing more is needed. There are no degrees of “thoroughness.” To say we need something more than the Bible as a rule of faith is to say we are made “partially thoroughly” equipped by the Bible and need something more to make us “completely and thoroughly” equipped.
Roman Catholicism adds to the Bible and expands the rule of faith. Catholics look to both “written books and unwritten traditions,” thereby adding to the Bible the declarations of popes and bishops, papal bulls, and various councils. Catholics believe the Bible is God’s Word, but it is incomplete without the addition of these writings of men. Mormonism’s rule of faith adds The Book of Mormon, Doctrine and Covenants, and The Pearl of Great Price to the Bible as equally inspired writings. This puts the opinions, views, and interpretations of men on a par with the Word of God.
Among Wesleyans, what is known as the “Quadrilateral” is the rule of faith: the Bible, tradition, reason, and experience. This, too, denies the sufficiency of Scripture. While Wesley himself may have not intended that the four rules of the Quadrilateral be seen as equal, by adding three sources of authority to the Scriptures, he opened the door for misinterpretation and misunderstanding. One can justify a belief in just about anything if that belief is based on tradition, reason, and/or experience. Jesus expressly forbade equating tradition with Scripture as part of the rule of faith, accusing the Pharisees of nullifying the Word of God for the sake of their traditions (Mark 7:6–13). Once the Word of God is rejected as the only rule of faith and practice, the door to heresy is opened and the genie is not easily put back into the bottle.
While it may be argued that the writers of the gospels and the epistles were offering their views and interpretations as they wrote, this is not the same as the declarations of the popes and bishops of Catholicism or of men like Joseph Smith. The New Testament writers were moved supernaturally by the Holy Spirit, who is the ultimate Author of Scripture. The biblical writers “spoke from God as they were carried along by the Holy Spirit” (2 Peter 1:21).
The Bible itself asserts that believers are indwelt by the Holy Spirit (1 Corinthians 6:19), who teaches, interprets, brings truths to mind (John 14:26), and guides us into all truth (John 16:13). For evangelical Christians, the Word of God and its Author are the true rule of faith. This is not to say that preachers and teachers are unnecessary. But the doctrine of the sufficiency of Scripture is a cornerstone of the evangelical rule of faith.