The rule of faith (regula fidei), indeed, is altogether one, alone immoveable and irreformable; the rule, to wit, of believing in one only God omnipotent, the Creator of the universe, and His Son Jesus Christ, born of the Virgin Mary, crucified under Pontius Pilate, raised again the third day from the dead, received in the heavens, sitting now at the right (hand) of the Father, destined to come to judge quick and dead through the resurrection of the flesh as well (as of the spirit).

Tertullian, “On the Veiling of Virgins,” in Fathers of the Third Century: Tertullian, Part Fourth; Minucius Felix; Commodian; Origen, Parts First and Second, ed. Alexander Roberts, James Donaldson, and A. Cleveland Coxe, trans. S. Thelwall, vol. 4, The Ante-Nicene Fathers (Buffalo, NY: Christian Literature Company, 1885), 27.

First we affirm that we desire to follow Scripture alone as rule of faith and religion, without mixing with it any other thing which might be devised by the opinion of men apart from the Word of God, and without wishing to accept for our spiritual government any other doctrine than what is conveyed to us by the same Word without addition or diminution, according to the command of our Lord.

Genevan Confession (1536), art. 1

But we hold that the interpretation of the Scripture to be orthodox and genuine which is gleaned from the Scriptures themselves (from the nature of the language in which they were written, likewise according to the circumstances in which they were set down, and expounded in the light of and unlike passages and of many and clearer passages) and which agree with the rule of faith (regula fidei) and love, and contributes much to the glory of God and man’s salvation.

Second Helvetic Confession (Heinrich Bullinger; 1566), ch. 2.

Under the name of Holy Scripture, or the Word of God written, are now contained all the books of the Old and New Testaments…All which are given by inspiration of God to be the rule of faith and life.

Westminster Confession of Faith 1.2.

All synods or councils, since the Apostles’ times, whether general or particular, may err; and many have erred. Therefore they are not to be made the rule of faith, or practice; but to be used as a help in both.

Westminster Confession of Faith 31.3

3. Q: What is the Word of God?

A. The Holy Scriptures of the Old and New Testament are the Word of God, the only rule of faith and obedience.

Westminster Larger Catechism, 3.